System and method for identifying and responding to P-wave oversensing in a cardiac system

ABSTRACT

A cardiac medical system, such as an implantable cardioverter defibrillator (ICD) system, receives a cardiac electrical signal by and senses cardiac events when the signal crosses an R-wave sensing threshold. The system determines at least one sensed event parameter from the cardiac electrical signal for consecutive cardiac events sensed by the sensing circuit and compares the sensed event parameters to P-wave oversensing criteria. The system detects P-wave oversensing in response to the sensed event parameters meeting the P-wave oversensing criteria; and adjusts at least one of an R-wave sensing control parameter or a therapy delivery control parameter in response to detecting the P-wave oversensing.

RELATED APPLICATIONS

This application claims the benefit of the filing date of a provisionalU.S. Application Ser. No. 62/347,177, filed Jun. 8, 2016, which isincorporated by reference herein in its entirety.

TECHNICAL FIELD

The disclosure relates generally to a cardiac system and a method foridentifying and responding to P-wave oversensing (PWOS).

BACKGROUND

Medical devices, such as cardiac pacemakers and ICDs, providetherapeutic electrical stimulation to a heart of a patient viaelectrodes carried by one or more medical electrical leads and/orelectrodes on a housing of the medical device. The electricalstimulation may include signals such as pacing pulses or cardioversionor defibrillation shocks. In some cases, a medical device may sensecardiac electrical signals attendant to the intrinsic or pacing-evokeddepolarizations of the heart and control delivery of stimulation signalsto the heart based on sensed cardiac electrical signals. Upon detectionof an abnormal rhythm, such as bradycardia, tachycardia or fibrillation,an appropriate electrical stimulation signal or signals may be deliveredto restore or maintain a more normal rhythm of the heart. For example,an ICD may deliver pacing pulses to the heart of the patient upondetecting bradycardia or tachycardia or deliver cardioversion ordefibrillation shocks to the heart upon detecting tachycardia orfibrillation. The ICD may sense the cardiac electrical signals in aheart chamber and deliver electrical stimulation therapies to the heartchamber using electrodes carried by transvenous medical electricalleads. Cardiac signals sensed within the heart generally have a highsignal strength and quality for reliably sensing cardiac electricalevents, such as R-waves. In other examples, a non-transvenous lead maybe coupled to the ICD, in which case cardiac signal sensing presents newchallenges in accurately sensing cardiac electrical events.

SUMMARY

In general, the disclosure is directed to techniques for identifyingP-wave oversensing by an implantable cardioverter defibrillator (ICD)and responding to the PWOS, for example by adjusting an R-wave sensingcontrol parameter and/or adjusting a therapy control parameter. An ICDoperating according to the techniques disclosed herein detects PWOSbased on analysis of a cardiac electrical signal received by anextra-cardiovascular sensing electrode vector. In some examples,clusters of sensed cardiac events are detected as evidence of PWOS.

In one example, the disclosure provides an extra-cardiovascular ICDsystem including a sensing circuit, a therapy delivery circuit and acontrol circuit. The sensing circuit is configured to receive a cardiacelectrical signal from electrodes coupled to the ICD and sense a cardiacevent in response to the cardiac electrical signal crossing an R-wavesensing threshold. The therapy delivery circuit is configured to deliverelectrical stimulation therapy to a patient's heart via electrodescoupled to the ICD. The control circuit is configured to determine atleast one sensed event parameter from the cardiac electrical signal foreach one of a plurality of consecutive cardiac events sensed by thesensing circuit, compare the sensed event parameters to P-waveoversensing criteria, detect P-wave oversensing in response to thesensed event parameters meeting the P-wave oversensing criteria, andadjust an R-wave sensing control parameter and/or a therapy deliverycontrol parameter in response to detecting the P-wave oversensing.

In another example, the disclosure provides a method performed by anextra-cardiovascular implantable cardioverter defibrillator (ICD)system. The method includes receiving a cardiac electrical signal by asensing circuit via electrodes coupled to the ICD, sensing a cardiacevent in response to the cardiac electrical signal crossing an R-wavesensing threshold, determining by a control circuit of the ICD at leastone sensed event parameter from the cardiac electrical signal for eachone of consecutive cardiac events sensed by the sensing circuit,comparing the sensed event parameters to P-wave oversensing criteria,detecting P-wave oversensing in response to the sensed event parametersmeeting the P-wave oversensing criteria; and adjusting at least one ofan R-wave sensing control parameter or a therapy delivery controlparameter in response to detecting the P-wave oversensing.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control circuit of an ICD system, cause the system toreceive a cardiac electrical signal by a sensing circuit via electrodescoupled to the ICD, sense a cardiac event in response to the cardiacelectrical signal crossing an R-wave sensing threshold, determine by acontrol circuit of the ICD at least one sensed event parameter from thecardiac electrical signal for each one of a plurality of consecutivecardiac events sensed by the sensing circuit, compare the sensed eventparameters to P-wave oversensing criteria, detect P-wave oversensing inresponse to the sensed event parameters meeting the P-wave oversensingcriteria, and adjust an R-wave sensing control parameter and/or atherapy delivery control parameter in response to detecting the P-waveoversensing.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with theextra-cardiovascular ICD system of FIG. 1A in a different implantconfiguration.

FIG. 3 is a conceptual diagram of a distal portion of anextra-cardiovascular lead having an electrode configuration according toanother example.

FIG. 4 is a schematic diagram of the ICD of FIGS. 1A-2C according to oneexample.

FIG. 5A is a conceptual diagram of R-wave sensed event signals that maybe produced by the sensing circuit of the ICD of FIG. 1 during PWOS.

FIG. 5B is a conceptual diagram of a bandpass filtered and rectifiedcardiac electrical signal depicting cardiac events and an R-wave sensingthreshold.

FIG. 5C is a conceptual diagram of a bandpass filtered and rectifiedcardiac electrical signal showing an example of PWOS when the R-waveamplitude is relatively small.

FIG. 6 is a conceptual diagram of a bandpass filtered and rectifiedcardiac electrical signal showing another example of PWOS.

FIG. 7 is a conceptual diagram of a bandpass filtered and rectifiedcardiac electrical signal showing yet another example of PWOS.

FIG. 8 is a flow chart of a method for identifying and responding toPWOS according to one example.

FIG. 9 is a flow chart of a method performed by the ICD of FIG. 1A fordetecting a sensed event cluster according to one example.

FIG. 10 is a flow chart of a method for analyzing clustered eventwaveforms for identifying a sensed event cluster as PWOS.

FIG. 11 is a flow chart of a method performed by the ICD of FIG. 1A foridentifying and responding to PWOS according to another example.

FIG. 12 is a flow chart of a method for identifying PWOS by an ICDaccording to another example.

FIG. 13 is a flow chart of a method for confirming R-waves afteridentifying PWOS.

FIG. 14 is a timing diagram of sensed event signals that may be producedby the ICD sensing circuit and received by the ICD control circuit andillustrates techniques for detecting PWOS.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for sensing cardiacelectrical signals using implanted, extra-cardiovascular electrodes. Asused herein, the term “extra-cardiovascular”refers to a position outsidethe blood vessels, heart, and pericardium surrounding the heart of apatient. Implantable electrodes carried by extra-cardiovascular leadsmay be positioned extra-thoracically (outside the ribcage and sternum)or intra-thoracically (beneath the ribcage or sternum) but generally notin intimate contact with myocardial tissue. The techniques disclosedherein provide a method for identifying PWOS in an extra-cardiovascularICD system. The term “P-wave oversensing” or “PWOS” as used hereinrefers to falsely sensing an R-wave by the sensing circuitry of anextra-cardiovascular ICD or pacemaker when an intrinsic P-wave occurs.P-waves, attendant to atrial depolarization, are typically small inamplitude relative to R-waves, attendant to ventricular depolarization,and therefore typically have a peak amplitude less than an R-wavesensing threshold and do not interfere with reliable R-wave sensing.When a cardiac electrical signal is being acquired usingextra-cardiovascular electrodes, however, PWOS can occur, particularlywhen the amplitude of the R-waves is relatively small or when the heartrate is slow. Reliable R-wave sensing is important in detectingventricular arrhythmias.

If P-waves are oversensed as R-waves, the heart rate may beover-estimated, leading to a false heart rhythm determination. Forexample, if the patient's heart rate is very slow and bradycardia pacingis required, PWOS may cause the ventricular rate to appear within anormal range to the ICD, resulting in withholding of bradycardia pacingpulses that may be needed to maintain a normal heart rate withouthemodynamic insufficiency. If the patient's heart is in a normal rangebut PWOS is occurring, the heart rate may appear faster than it actuallyis, and the ICD may detect a ventricular tachyarrhythmia, which may leadto unnecessary tachyarrhythmia therapy being delivered, such asanti-tachycardia pacing (ATP) or one or morecardioversion/defibrillation shocks. The techniques disclosed herein foridentifying PWOS enables identified PWOS to be rejected or ignored indetermining a heart rhythm so that the ICD may provide an appropriatetherapy delivery response.

The PWOS detection techniques are described in conjunction with an ICDcoupled to implantable medical lead carrying extra-cardiovascularelectrodes used for sensing cardiac electrical signals. Aspectsdisclosed herein for identifying and responding to PWOS, however, may beutilized in conjunction with a variety of implantable or externaldevices that utilize other cardiac electrical sensing lead or electrodesystems. For example, the techniques for PWOS as described inconjunction with the accompanying drawings may be implemented in anyimplantable or external medical device enabled for sensing cardiacelectrical signals, including implantable pacemakers, ICDs, CRT-Ps,CRT-Ds or cardiac monitors coupled to transvenous or epicardial leadscarrying sensing electrodes; leadless pacemakers, ICDs, CRT-Ps, CRT-Dsor cardiac monitors having housing-based sensing electrodes; andexternal pacemakers, defibrillators, or cardiac monitors coupled toexternal, surface or skin electrodes.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 according to one example. FIG. 1A is a front view of ICDsystem 10 implanted within patient 12. ICD system 10 includes an ICD 14connected to an extra-cardiovascular electrical stimulation and sensinglead 16. FIG. 1B is a side view of the distal portion 25 of lead 16implanted within patient 12. FIGS. 1A and 1B are described in thecontext of an ICD system 10 capable of providing defibrillation and/orcardioversion shocks and pacing pulses.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as a housing electrode (sometimes referred to as a canelectrode). In examples described herein, housing 15 may be used as anactive can electrode for use in delivering cardioversion/defibrillation(CV/DF) shocks or other high voltage pulses delivered using a highvoltage therapy circuit. In other examples, housing 15 may be availablefor use in delivering unipolar, low voltage cardiac pacing pulses inconjunction with lead-based cathode electrodes. In other instances, thehousing 15 of ICD 14 may include a plurality of electrodes on an outerportion of the housing. The outer portion(s) of the housing 15functioning as an electrode(s) may be coated with a material, such astitanium nitride.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,sensors, electrical cardiac signal sensing circuitry, therapy deliverycircuitry, power sources and other components for sensing cardiacelectrical signals, detecting a heart rhythm, and controlling anddelivering electrical stimulation pulses to treat an abnormal heartrhythm.

Lead 16 includes an elongated lead body 18 having a proximal end 27 thatincludes a lead connector (not shown) configured to be connected to ICDconnector assembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead 16 includes defibrillation electrodes 24 and 26 andpace/sense electrodes 28, 30 and 31. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently. In some instances, defibrillation electrodes 24and 26 are coupled to electrically isolated conductors, and ICD 14 mayinclude switching mechanisms to allow electrodes 24 and 26 to beutilized as a single defibrillation electrode (e.g., activatedconcurrently to form a common cathode or anode) or as separatedefibrillation electrodes, (e.g., activated individually, one as acathode and one as an anode or activated one at a time, one as an anodeor cathode and the other remaining inactive with housing 15 as an activeelectrode).

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to low voltage pacing and sensing electrodes 28, 30 and31. However, electrodes 24 and 26 and housing 15 may also be utilized toprovide pacing functionality, sensing functionality or both pacing andsensing functionality in addition to or instead of high voltagestimulation therapy. In this sense, the use of the term “defibrillationelectrode” herein should not be considered as limiting the electrodes 24and 26 for use in only high voltage cardioversion/defibrillation shocktherapy applications. Electrodes 24 and 26 may be used in a pacingelectrode vector for delivering extra-cardiovascular pacing pulses, suchas ATP pulses, post-shock pacing pulses or bradycardia pacing pulses,and/or in a sensing electrode vector used to sense cardiac electricalsignals and detect ventricular tachycardia (VT) and ventricularfibrillation (VF).

Electrodes 28, 30 and 31 are relatively smaller surface area electrodes(compared to defibrillation electrodes 24 and 26) for delivering lowvoltage pacing pulses and for sensing cardiac electrical signals.Electrodes 28, 30 and 31 are referred to as pace/sense electrodesbecause they are generally configured for use in low voltageapplications, e.g., used as either a cathode or anode for delivery ofpacing pulses and/or sensing of cardiac electrical signals. In someinstances, electrodes 28, 30 and 31 may provide only pacingfunctionality, only sensing functionality or both.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. A third pace/senseelectrode 31 may optionally be located distal to defibrillationelectrode 26. In other examples, one or more pace/sense electrodes maybe carried by lead 16 and be located proximal to defibrillationelectrode 24, between defibrillation electrodes 24 and 26, and/or distalto defibrillation electrode 26.

Electrodes 28 and 30 are illustrated as ring electrodes, and electrode31 is illustrated as a hemispherical tip electrode in the example ofFIGS. 1A and 1B. However, electrodes 28, 30 and 31 may comprise any of anumber of different types of electrodes, including ring electrodes,short coil electrodes, hemispherical electrodes, directional electrodes,segmented electrodes, or the like, and may be positioned at any positionalong the distal portion 25 of lead 16. Further, electrodes 28, 30 and31 may be of similar type, shape, size and material or may differ fromeach other.

Lead 16 extends subcutaneously or submuscularly over the ribcage 32medially from the connector assembly 27 of ICD 14 toward a center of thetorso of patient 12, e.g., toward xiphoid process 20 of patient 12. At alocation near xiphoid process 20, lead 16 bends or turns and extendssuperior subcutaneously or submuscularly over the ribcage and/orsternum, substantially parallel to sternum 22. Although illustrated inFIGS. 1A and 1B as being offset laterally from and extendingsubstantially parallel to sternum 22, lead 16 may be implanted at otherlocations, such as over sternum 22, offset to the right or left ofsternum 22, angled laterally from sternum 22 toward the left or theright, or the like. Alternatively, lead 16 may be placed along othersubcutaneous or submuscular paths. The path of lead 16 may depend on thelocation of ICD 14, the arrangement and position of electrodes carriedby the lead distal portion 25, and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, 30 and 31 locatedalong the distal portion 25 of the lead body 18. Lead body 18 may betubular or cylindrical in shape. In other examples, the distal portion25 (or all of) the elongated lead body 18 may have a flat, ribbon orpaddle shape. The lead body 18 of lead 16 may be formed from anon-conductive material, including silicone, polyurethane,fluoropolymers, mixtures thereof, and other appropriate materials, andshaped to form one or more lumens within which the one or moreconductors extend. However, the techniques disclosed herein are notlimited to such constructions or to any particular lead body design.

The elongated electrical conductors contained within the lead body 18are each electrically coupled with respective defibrillation electrodes24 and 26 and pace/sense electrodes 28, 30 and 31. Each of pacing andsensing electrodes 28, 30 and 31 are coupled to respective electricalconductors, which may be separate respective conductors within the leadbody. The respective conductors electrically couple the electrodes 24,26, 28, 30 and 31 to circuitry of ICD 14, such as a therapy deliverycircuit and/or a sensing circuit as described below, via connections inthe connector assembly 17, including associated electrical feedthroughscrossing housing 15. The electrical conductors transmit therapy from atherapy circuit within ICD 14 to one or more of defibrillationelectrodes 24 and 26 and/or pace/sense electrodes 28, 30 and 31 andtransmit sensed electrical signals from one or more of defibrillationelectrodes 24 and 26 and/or pace/sense electrodes 28, 30 and 31 to thesensing circuit within ICD 14.

ICD 14 may obtain electrical signals corresponding to electricalactivity of heart 8 via a combination of sensing vectors that includecombinations of electrodes 28, 30, and/or 31. In some examples, housing15 of ICD 14 is used in combination with one or more of electrodes 28,30 and/or 31 in a sensing electrode vector. ICD 14 may even obtaincardiac electrical signals using a sensing electrode vector thatincludes one or both defibrillation electrodes 24 and/or 26, e.g.,between electrodes 24 and 26 or one of electrodes 24 or 26 incombination with one or more of electrodes 28, 30, 31, and/or thehousing 15.

ICD 14 analyzes the cardiac electrical signals received from one or moreof the sensing vectors to monitor for abnormal rhythms, such asbradycardia, ventricular tachycardia (VT) or ventricular fibrillation(VF). ICD 14 may analyze the heart rate and/or morphology of the cardiacelectrical signals to monitor for tachyarrhythmia in accordance with anyof a number of tachyarrhythmia detection techniques. One exampletechnique for detecting tachyarrhythmia is described in U.S. Pat. No.7,761,150 (Ghanem, et al.), incorporated herein by reference in itsentirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF). ICD 14 may deliver ATPin response to VT detection, and in some cases may deliver ATP prior toa CV/DF shock or during high voltage capacitor charging in an attempt toavert the need for delivering a CV/DF shock. ATP may be delivered usingan extra-cardiovascular pacing electrode vector selected from any ofelectrodes 24, 26, 28, 30, 31 and/or housing 15. The pacing electrodevector may be different than the sensing electrode vector. In oneexample, cardiac electrical signals are sensed between pace/senseelectrodes 28 and 30, and ATP pulses and other pacing pulses aredelivered between pace/sense electrode 30 used as a cathode electrodeand defibrillation electrode 24 used as a return anode electrode. Inother examples, pacing pulses may be delivered between pace/senseelectrode 28 and either (or both) defibrillation electrode 24 or 26 orbetween defibrillation electrode 24 and defibrillation electrode 26.These examples are not intended to be limiting, and it is recognizedthat other sensing electrode vectors and pacing electrode vectors may beselected according to individual patient need.

If ATP does not successfully terminate VT or when VF is detected, ICD 14may deliver one or more cardioversion or defibrillation (CV/DF) shocksvia one or both of defibrillation electrodes 24 and 26 and/or housing15. ICD 14 may deliver the CV/DF shocks using electrodes 24 and 26individually or together as a cathode (or anode) and with the housing 15as an anode (or cathode). ICD 14 may generate and deliver other types ofelectrical stimulation pulses such as post-shock pacing pulses orbradycardia pacing pulses using a pacing electrode vector that includesone or more of the electrodes 24, 26, 28, 30 and 31 and the housing 15of ICD 14.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. Variousexample configurations of extra-cardiovascular leads and electrodes anddimensions that may be implemented in conjunction with theextra-cardiovascular sensing techniques disclosed herein are describedin U.S. Publication No. 2015/0306375 (Marshall, et al.) and U.S.Publication No. 2015/0306410 (Marshall, et al.), both of which areincorporated herein by reference in their entirety.

ICD 14 is shown implanted subcutaneously on the left side of patient 12along the ribcage 32. ICD 14 may, in some instances, be implantedbetween the left posterior axillary line and the left anterior axillaryline of patient 12. ICD 14 may, however, be implanted at othersubcutaneous or submuscular locations in patient 12. For example, ICD 14may be implanted in a subcutaneous pocket in the pectoral region. Inthis case, lead 16 may extend subcutaneously or submuscularly from ICD14 toward the manubrium of sternum 22 and bend or turn and extendinferior from the manubrium to the desired location subcutaneously orsubmuscularly. In yet another example, ICD 14 may be placed abdominally.Lead 16 may be implanted in other extra-cardiovascular locations aswell. For instance, as described with respect to FIGS. 2A-2C, the distalportion 25 of lead 16 may be implanted underneath the sternum/ribcage inthe substernal space.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with ICD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenICD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) orother RF or communication frequency bandwidth.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may alternatively be embodied as a homemonitor or hand held device. External device 40 may be used to programcardiac rhythm detection parameters and therapy control parameters usedby ICD 14. Control parameters used to identify PWOS according totechniques disclosed herein may be programmed into ICD 14 using externaldevice 40. Data stored or acquired by ICD 14, including physiologicalsignals or associated data derived therefrom, results of devicediagnostics, and histories of detected rhythm episodes and deliveredtherapies, may be retrieved from ICD 14 by external device 40 followingan interrogation command.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement, lead 16 ofsystem 10 is implanted at least partially underneath sternum 22 ofpatient 12. Lead 16 extends subcutaneously or submuscularly from ICD 14toward xiphoid process 20 and at a location near xiphoid process 20bends or turns and extends superiorly within anterior mediastinum 36 ina substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22.In some instances, the anterior wall of anterior mediastinum 36 may alsobe formed by the transversus thoracis muscle and one or more costalcartilages. Anterior mediastinum 36 includes a quantity of looseconnective tissue (such as areolar tissue), adipose tissue, some lymphvessels, lymph glands, substernal musculature, small side branches ofthe internal thoracic artery or vein, and the thymus gland. In oneexample, the distal portion 25 of lead 16 extends along the posteriorside of sternum 22 substantially within the loose connective tissueand/or substernal musculature of anterior mediastinum 36.

A lead implanted such that the distal portion 25 is substantially withinanterior mediastinum 36 may be referred to as a “substernal lead.” Inthe example illustrated in FIGS. 2A-2C, lead 16 is located substantiallycentered under sternum 22. In other instances, however, lead 16 may beimplanted such that it is offset laterally from the center of sternum22. In some instances, lead 16 may extend laterally such that distalportion 25 of lead 16 is underneath/below the ribcage 32 in addition toor instead of sternum 22. In other examples, the distal portion 25 oflead 16 may be implanted in other extra-cardiovascular, intra-thoraciclocations, including the pleural cavity or around the perimeter of andadjacent to but typically not within the pericardium 38 of heart 8.Other implant locations and lead and electrode arrangements that may beused in conjunction with the techniques described herein are generallydisclosed in the above-incorporated patent applications.

FIG. 3 is a conceptual diagram illustrating a distal portion 25′ ofanother example of extra-cardiovascular lead 16 of FIGS. 1A-2C having acurving distal portion 25′ of lead body 18′. Lead body 18′ may be formedhaving a curving, bending, serpentine, or zig-zagging shape along distalportion 25′. In the example shown, defibrillation electrodes 24′ and 26′are carried along curving portions of the lead body 18′. Pace/senseelectrode 30′ is carried in between defibrillation electrodes 24′ and26′. Pace/sense electrode 28′ is carried proximal to the proximaldefibrillation electrode 24′. No electrode is provided distal todefibrillation electrode 26′ in this example.

As shown in FIG. 3, lead body 18′ may be formed having a curving distalportion 25′ that includes two “C” shaped curves, which together mayresemble the Greek letter epsilon, “ε.” Defibrillation electrodes 24′and 26′ are each carried by one of the two respective C-shaped portionsof the lead body distal portion 25′, which extend or curve in the samedirection away from a central axis 33 of lead body 18′. In the exampleshown, pace/sense electrode 28′ is proximal to the C-shaped portioncarrying electrode 24′, and pace/sense electrode 30′ is proximal to theC-shaped portion carrying electrode 26′. Pace/sense electrodes 28′ and30′ may, in some instances, be approximately aligned with the centralaxis 33 of the straight, proximal portion of lead body 18′ such thatmid-points of defibrillation electrodes 24′ and 26′ are laterally offsetfrom electrodes 28′ and 30′. Other examples of extra-cardiovascularleads including one or more defibrillation electrodes and one or morepacing and sensing electrodes carried by curving, serpentine, undulatingor zig-zagging distal portion of the lead body that may be implementedwith the pacing techniques described herein are generally disclosed inU.S. patent application Ser. No. 14/963,303, incorporated herein byreference in its entirety.

FIG. 4 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 4) includes software, firmware and hardware thatcooperatively monitor one or more cardiac electrical signals, determinewhen an electrical stimulation therapy is necessary, and delivertherapies as needed according to programmed therapy delivery algorithmsand control parameters. The software, firmware and hardware areconfigured to detect ventricular tachyarrhythmia and may discriminate VTand VF for determining when ATP or CV/DF shocks are required. In somecases, ICD 14 may be configured to deliver bradycardia pacing when theheart rate falls below a programmed lower rate. ICD 14 is coupled to anextra-cardiovascular lead, such as lead 16 shown in FIG. 1A or lead 16′of FIG. 3, carrying extra-cardiovascular electrodes 24, 26, 28, 30 and31 (if available), for delivering electrical stimulation pulses to thepatient's heart and for sensing cardiac electrical signals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, sensing circuit 86, and telemetry circuit 88. A power source98 provides power to the circuitry of ICD 14, including each of thecomponents 80, 82, 84, 86, and 88 as needed. Power source 98 may includeone or more energy storage devices, such as one or more rechargeable ornon-rechargeable batteries. The connections between power source 98 andeach of the other components 80, 82, 84, 86 and 88 are to be understoodfrom the general block diagram of FIG. 4, but are not shown for the sakeof clarity. For example, power source 98 may be coupled to a low voltage(LV) charging circuit and to a high voltage (HV) charging circuitincluded in therapy delivery circuit 84 for charging low voltage andhigh voltage capacitors, respectively, included in therapy deliverycircuit 84 for producing respective low voltage pacing pulses, such asbradycardia pacing, post-shock pacing or ATP pulses, or for producinghigh voltage pulses, such as CV/DF shock pulses. In some examples, highvoltage capacitors are charged and utilized for delivering cardiacpacing pulses, ATP and/or post-shock pacing pulses instead of lowvoltage capacitors.

The functional blocks shown in FIG. 4 represent functionality includedin ICD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to ICD 14 herein. The variouscomponents may include an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsthat provide the described functionality. The particular form ofsoftware, hardware and/or firmware employed to implement thefunctionality disclosed herein will be determined primarily by theparticular system architecture employed in the ICD and by the particulardetection and therapy delivery methodologies employed by the ICD.Providing software, hardware, and/or firmware to accomplish thedescribed functionality in the context of any modern ICD system, giventhe disclosure herein, is within the abilities of one of skill in theart.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such as arandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control circuit 80 orother ICD components to perform various functions attributed to ICD 14or those ICD components. The non-transitory computer-readable mediastoring the instructions may include any of the media listed above.

The functions attributed to ICD 14 herein may be embodied as one or moreintegrated circuits. Depiction of different features as components(e.g., circuits) is intended to highlight different functional aspectsand does not necessarily imply that such components (e.g., circuits ormodules) must be realized by separate hardware or software components.Rather, functionality associated with one or more components may beperformed by separate hardware, firmware or software components, orintegrated within common hardware, firmware or software components. Forexample, sensing operations may be performed by sensing circuit 86 underthe control of control circuit 80 and identification of PWOS operationsmay be implemented in a processor of control circuit 80 executinginstructions stored in memory 82.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and sensing circuit 86 for sensing cardiacelectrical activity, detecting cardiac rhythms, and controlling deliveryof cardiac electrical stimulation therapies in response to sensedcardiac signals. Therapy delivery circuit 84 and sensing circuit 86 areelectrically coupled to electrodes 24, 26, 28, and 30 31 carried by lead16 and the housing 15, which may function as a common or groundelectrode or as an active can electrode for delivering CV/DF shockpulses or cardiac pacing pulses.

Sensing circuit 86 may be selectively coupled to electrodes 28, 30, 31and/or housing 15 in order to monitor electrical activity of thepatient's heart. Sensing circuit 86 may additionally be selectivelycoupled to defibrillation electrodes 24 and/or 26 for use in a sensingelectrode vector. Sensing circuit 86 is enabled to selectively monitorone or more sensing vectors at a time selected from the availableelectrodes 24, 26, 28, 30, 31 and housing 15. For example, sensingcircuit 86 may include switching circuitry for selecting which ofelectrodes 24, 26, 28, 30, 31 and housing 15 are coupled to senseamplifiers or other cardiac event detection circuitry included in one ormore sensing channels of sensing circuit 86. Switching circuitry mayinclude a switch array, switch matrix, multiplexer, or any other type ofswitching device suitable to selectively couple components of sensingcircuit 86 to selected electrodes. In some instances, control circuit 80may control the switching circuitry to selectively couple sensingcircuit 86 to one or more sense electrode vectors. The cardiac eventdetection circuitry within sensing circuit 86 may include one or moresense amplifiers, filters, rectifiers, threshold detectors, comparators,analog-to-digital converters (ADCs), or other analog or digitalcomponents.

In some examples, sensing circuit 86 includes multiple sensing channelsfor acquiring cardiac electrical signals from multiple sensing vectorsselected from electrodes 24, 26, 28, 30, 31 and housing 15. Each sensingchannel may be configured to amplify, filter and rectify the cardiacelectrical signal received from selected electrodes coupled to therespective sensing channel to improve the signal quality for sensingcardiac events, such as R-waves. For example, each sensing channel mayinclude a pre-filter and amplifier for filtering and amplifying a signalreceived from a selected pair of electrodes. The resulting raw cardiacelectrical signal may be passed from the pre-filter and amplifier tocardiac event detection circuitry for sensing cardiac events from thereceived cardiac electrical signal. Cardiac event detection circuitrymay include a pre-filter and amplifier, an analog-to-digital converter,a bandpass filter, a rectifier, a sense amplifier and/or comparator fordetecting a cardiac event when the cardiac electrical signal crosses asensing threshold. For example, an R-wave sensing threshold may beautomatically adjusted by sensing circuit 86 under the control ofcontrol circuit 80. The R-wave sensing threshold may have a startingthreshold value set as a percentage of a maximum peak amplitude of theimmediately preceding sensed R-wave. The R-wave sensing threshold maydecrease from a starting value in a decaying or step-wise manner overpredetermined time intervals. Parameters used to determine and controlthe R-wave sensing threshold values may be stored in memory 82 andcontrolled by hardware or firmware of control circuit 80 and/or sensingcircuit 86. Some sensing threshold control parameters may be programmedby a user and passed from control circuit 80 to sensing circuit 86 via adata bus.

Sensing circuit 86 may sense R-waves according to a sensing thresholdthat is automatically adjusted. For example, the R-wave sensingthreshold may decay from a starting percentage, e.g., 60% of the maximumpeak amplitude of the most recently sensed R-wave. In other examples,the R-wave sensing threshold may be adjusted in a step-wise manner tomultiple threshold levels at specified times after a sensing thresholdcrossing as disclosed in U.S. patent application Ser. No. 15/142,171 ,incorporated herein by reference in its entirety.

Upon detecting a cardiac event based on a sensing threshold crossing,sensing circuit 86 may produce a sensed event signal, such as an R-wavesensed event signal, that is passed to control circuit 80. The sensedevent signals are used by control circuit 80 for detecting cardiacrhythms and determining a need for therapy. Sensing circuit 86 may alsopass a digitized cardiac electrical signal to control circuit 80 forwaveform morphology analysis performed for detecting and discriminatingheart rhythms.

Signals from the selected sensing vector may be passed through abandpass filter and amplifier, provided to a multiplexer and convertedto multi-bit digital signals by an analog-to-digital converter, allincluded in sensing circuit 86, for storage in random access memoryincluded in memory 82 under control of a direct memory access circuitvia a data/address bus. Control circuit 80 may be a microprocessor-basedcontroller that employs digital signal analysis techniques tocharacterize the digitized signals stored in random access memory ofmemory 82 to recognize and classify the patient's heart rhythm employingany of numerous signal processing methodologies for analyzing cardiacsignals and cardiac event waveforms, e.g., R-waves. Examples ofalgorithms that may be performed by ICD 14 for detecting, discriminatingand treating tachyarrhythmia which may be adapted to include PWOSidentification techniques described herein are generally disclosed inU.S. Pat. No. 5,354,316 (Keimel); U.S. Pat. No. 5,545,186 (Olson, etal.); U.S. Pat. No. 6,393,316 (Gillberg et al.); U.S. Pat. No. 7,031,771(Brown, et al.); U.S. Pat. No. 8,160,684 (Ghanem, et al.), and U.S. Pat.No. 8,437,842 (Zhang, et al.), all of which patents are incorporatedherein by reference in their entirety.

Therapy delivery circuit 84 includes charging circuitry; one or morecharge storage devices, such as one or more high voltage capacitors andin some examples one or more low voltage capacitors, and switchingcircuitry that controls when the capacitor(s) are discharged across aselected pacing electrode vector or CV/DF shock vector. Charging ofcapacitors to a programmed pulse amplitude and discharging of thecapacitors for a programmed pulse width may be performed by therapydelivery circuit 84 according to control signals received from controlcircuit 80. Control circuit 80 may include various timers or countersthat control when ATP or other cardiac pacing pulses are delivered.

For example, control circuit 80 may include pacer timing and controlcircuitry having programmable digital counters set by the microprocessorof the control circuit 80 for controlling the basic time intervalsassociated with various pacing modes or anti-tachycardia pacingsequences delivered by ICD 14. The microprocessor of control circuit 80may also set the amplitude, pulse width, polarity or othercharacteristics of the cardiac pacing pulses, which may be based onprogrammed values stored in memory 82.

During pacing, escape interval counters within the pacer timing andcontrol circuitry are reset upon sensing of R-waves as indicated bysignals from sensing circuit 86. In accordance with the selected mode ofpacing, pacing pulses are generated by a pulse output circuit of therapydelivery circuit 84 upon expiration of an escape interval counter. Thepace output circuit is coupled to the desired electrodes via switchmatrix for discharging one or more capacitors across the pacing load.The escape interval counters are reset upon generation of pacing pulses,and thereby control the basic timing of cardiac pacing functions,including anti-tachycardia pacing, bradycardia pacing, or post-shockpacing. The durations of the escape intervals are determined by controlcircuit 80 via a data/address bus. The value of the count present in theescape interval counters when reset by sensed R-waves can be used tomeasure RR intervals (RRIs) for detecting the occurrence of a variety ofarrhythmias. An RRI is the time interval between two consecutivelysensed R-waves.

As described below, control circuit 80 may monitor RRIs for detecting anRRI pattern that is evidence of PWOS. The pattern may be a pattern ofalternating long and short RRIs, e.g., long-short-long-short, orclusters of sensed events occurring at short intervals separated by onelong interval, e.g., short-short-long or short-short-short-long. When apattern of RRIs that is indicative of PWOS is detected, e.g., when apredetermined number of sensed event clusters are detected based on RRIcriteria, the waveforms of the digitized cardiac electrical signalcorresponding to the RRI pattern of clustered sensed events may beanalyzed by control circuit 80 for identifying PWOS. PWOS may beidentified when RRI patterns and waveform morphology analysis meet PWOSdetection criteria. If PWOS is identified, and a tachyarrhythmia isbeing detected, the tachyarrhythmia episode detection may be rejected ora scheduled tachyarrhythmia therapy may be canceled or withheld. If PWOSis identified, and a tachyarrhythmia is not being detected, analysis ofsensed events, e.g., on a beat-by-beat basis, may be enabled to allowoversensed P-waves to be identified as they occur and ignored for thepurposes of resetting pacing escape interval counters so that anoversensed P-wave does not inhibit pacing pulse delivery. Additionallyor alternatively, if PWOS is identified, a cardiac signal segment may bestored and/or R-wave sensing control parameters may be adjusted toreduce the likelihood of PWOS in the future.

Memory 82 includes read-only memory (ROM) or other memory devices inwhich stored programs controlling the operation of the control circuit80 reside. Memory 82 may further include random access memory (RAM) orflash memory configured as a number of recirculating buffers capable ofholding a series of measured RRIs, cardiac signal segments, counts orother data for analysis by the control circuit 80 for predicting ordiagnosing an arrhythmia.

In response to the detection of ventricular tachycardia, ATP may bedelivered by loading a regimen from a microprocessor included in controlcircuit 80 into the pacer timing and control circuit according to thetype and rate of tachycardia detected. In the event that the tachycardiais not terminated by ATP or if VF is detected and higher voltagecardioversion or defibrillation pulses are required, the control circuitactivates cardioversion and defibrillation control circuitry included incontrol circuit 80 to initiate charging of the high voltage capacitorsvia a charging circuit, both included in therapy delivery circuit 84,under the control of a high voltage charging control line. The voltageon the high voltage capacitors is monitored via a voltage capacitorline, which is passed to control circuit 80. When the voltage reaches apredetermined value set by the microprocessor of control circuit 80, alogic signal is generated on a capacitor full line passed to therapydelivery circuit 84, terminating charging. The defibrillation orcardioversion pulse is delivered to the heart under the control of thepacer timing and control circuitry by an output circuit of therapydelivery circuit 84 via a control bus. The output circuit, which mayinclude multiple switches and may be in the form of an H-bridge circuit,determines the electrodes used for delivering the cardioversion ordefibrillation pulse and the pulse wave shape. Therapy delivery chargingand output circuity and control circuitry generally disclosed in any ofthe above-incorporated patents may be implemented in ICD 14.

Control parameters utilized by control circuit 80 for detecting cardiacrhythms and controlling therapy delivery may be programmed into memory82 via telemetry circuit 88. Telemetry circuit 88 includes a transceiverand antenna for communicating with external device 40 (shown in FIG. 1A)using RF communication as described above. Under the control of controlcircuit 80, telemetry circuit 88 may receive downlink telemetry from andsend uplink telemetry to external device 40. In some cases, telemetrycircuit 88 may be used to transmit and receive communication signalsto/from another medical device implanted in patient 12.

FIG. 5A is a conceptual diagram 100 of R-wave sensed event signals 102that may be produced by sensing circuit 86 during PWOS. Events 102 arelabeled as “P” for P-wave, “R” for R-wave and “r” designates adouble-sensed R-wave in which an uppercase “R” immediately followed by alowercase “r” represent sensed event signals produced by sensing thesame QRS complex twice. When extra-cardiovascular electrodes are used toacquire a cardiac electrical signal, the QRS waveform may be relativelywide in some patient's compared to the QRS waveform of a signal receivedusing transvenous, intracardiac electrodes or epicardial electrodes. Assuch, there may be a higher likelihood of double sensing an R-wave whena single R-wave signal exceeds the R-wave sensing threshold a secondtime outside of a blanking period.

The second time the R-wave is sensed the cardiac signal peak amplitudemay be lower than the actual peak amplitude of the R-wave. When theR-wave sensing threshold is set as a percentage of the maximum R-waveamplitude, a decaying or decreasing R-wave sensing threshold starting ata relatively low amplitude may be crossed by a subsequent P-wave. Inother cases, the R-wave sensed by extra-cardiovascular electrodes may below in amplitude in some instances. Relatively small amplitude R-wavesmay still be larger than P-waves or T-waves and appropriately sensed asR-waves. However, when the starting R-wave sensing threshold is set as apercentage of the maximum peak amplitude of a sensed R-wave, thestarting threshold may be set relatively low when the R-wave peakamplitude is low. A P-wave may be oversensed following the low-amplitudeR-wave in some instances, particularly when the heart rate is low.

FIG. 5B is a conceptual diagram of a bandpass filtered and rectifiedcardiac electrical signal 150 including R-wave 152 followed by a T-wave154 and P-wave 156, with no PWOS. The R-wave sensing threshold 160 isset to a starting threshold value 166 following a blanking period 172after R-wave 152 is sensed. R-wave 152 is sensed at time 171 andblanking period 172 is started.

During blanking period 172, the maximum peak amplitude 164 of R-wave 152is determined and used to set starting R-wave sensing threshold value166, e.g., approximately 60% of the maximum peak amplitude 164. R-wavesensing threshold 160 decreases over time from the starting value 166.In the example shown, R-wave sensing threshold 160 is shown to decreaseaccording to a predetermined decay rate 168 from the starting value 166up to the expiration of a predetermined time interval 173, or until aminimum sensing threshold value 170 is reached, whichever occurs first.If time interval 173 expires, the R-wave sensing threshold 160 drops tothe minimum sensing threshold value 170, which may be equal to aprogrammed sensitivity setting. In other examples, R-wave sensingthreshold 160 may decrease linearly and/or according to one or morestep-wise drops or according to other R-wave sensing threshold controlparameters, including the multi-level R-wave sensing threshold examplesdisclosed in the above-incorporated U.S. patent application Ser. No.15/142,171.

When the R-wave amplitude 164 is relatively high, the starting value 166based on amplitude 164 is relatively high such that the decaying R-wavesensing threshold 160 remains greater than the amplitude of P-wave 156(and T-wave 154) such that PWOS does not occur. The next R-wave 153 issensed at time 175 when the cardiac electrical signal crosses the R-wavesensing threshold 160, which has been reduced to the minimum value 170.The time interval 165 is determined as the RRI between consecutivelysensed R-waves 152 and 153 and may be used by control circuit 80 indetecting patterns of PWOS. For example, RRI 165 may represent a longRRI in an analysis of RRIs performed for detecting a pattern includinglong and short RRIs that may be evidence of PWOS.

In FIG. 5C, R-wave 158 is followed by a T-wave and an oversensed P-wave162. When a relatively small amplitude R-wave 158 occurs, or the ratioof the R-wave amplitude 174 to the P-wave amplitude 184 is relativelysmall, the starting R-wave sensing threshold 176 based on R-wave maximumpeak amplitude 174 is relatively low compared to the P-wave maximum peakamplitude 184. As R-wave sensing threshold decreases from the startingvalue 176 according to decay rate 168, it falls below the amplitude 184of P-wave 162. P-wave 162 is oversensed at time 178, causing sensingcircuit 86 to produce a false R-wave sensed event signal. The R-wavesensing threshold 160 is set to a starting value 186 based on the peakamplitude 184 of P-wave 162, falsely detected as an R-wave, resulting inthe R-wave sensing threshold returning to the minimum sensing threshold170 quickly as it decreases at decay rate 168.

The time interval 179 between the R-wave 158 sensed at time 177 and theP-wave 162 sensed at time 178 is determined as an RRI, but is arelatively short RRI and may be identified by control circuit 80 in apattern of RRIs that is evidence of PWOS. In the presence of an atrialtachyarrhythmia, such as atrial fibrillation or atrial flutter, multipleP-waves may occur between R-waves. When P-wave 162 is oversensed and isfollowed by a relatively low R-wave sensing threshold that is based onthe low peak amplitude 184 of P-wave 162, multiple P-waves may beoversensed sequentially leading to a cluster of sensed events occurringat short intervals.

As shown by FIG. 5C, when a relatively small amplitude R-wave 158occurs, subsequent PWOS may occur, particularly when the R-wave sensingthreshold starting value is set based on the maximum peak amplitude ofthe most recent sensed event. PWOS may occur repetitively until arelatively larger amplitude R-wave is sensed, which resets the R-wavesensing threshold to a proportionally higher starting value. The higherstarting value may maintain the R-wave sensing threshold above theP-wave amplitude and interrupt the sequence of sensed events thatinclude PWOS.

Relatively small R-waves may occur during a normal or fast ventricularrate, leading to PWOS and an overestimation of the ventricular rate. VTor VF may be falsely detected resulting in an unneeded therapy, such asan unnecessary cardioversion/defibrillation shock. Small R-waves mayalso occur during a slow ventricular rate leading to PWOS and anoverestimation of the ventricular rate when the patient may beexperiencing bradycardia. In this case, appropriate bradycardia pacingmay be withheld due to the PWOS because R-wave sensed event signalscause the pacing escape interval to be reset prior to expiration andpacing pulse delivery. As such, PWOS may lead to false detection ofventricular tachyarrhythmia and unnecessary therapy, and PWOS may leadto missed detection of bradycardia intervals and withholding of anappropriate bradycardia pacing therapy.

FIG. 6 is a conceptual diagram of a bandpass filtered and rectifiedcardiac electrical signal 250 including R-waves 252 and 260 withintervening T-wave 254 and P-wave 256. In this example, double sensingof R-wave 252 and oversensing of P-wave 256 occur resulting in a clusterof sensed events at times 251, 253, 259 and 261 occurring at relativelyshort RRIs 265, 267, and 269, respectively.

R-wave 252 is sensed first at time 251, and blanking period 172 isstarted. The R-wave sensing threshold 160 is set to a starting thresholdvalue 262 following blanking period 172. R-wave sensing threshold 160decays at decay rate 168. R-wave 252 has a relatively wide signal widthin this example such that when the first blanking interval 172 startedat time 251 expires, the rectified R-wave 252 crosses the R-wave sensingthreshold 160 a second time at time 253 outside blanking interval 172,starting another blanking interval 172. Control circuit 80 determines anRRI 265 between R-wave sensed event signals received at time 251 andtime 253.

The R-wave sensing threshold 160 is set to a starting value 264 based ona percentage of the maximum peak amplitude 257 detected during theblanking interval 172 started at time 253 corresponding to the secondtime R-wave 252 is sensed. The starting value 264 of R-wave sensingthreshold 160 is lower than the starting threshold 262 that is based ona percentage of the true maximum peak amplitude 255 of R-wave 252.R-wave sensing threshold 160 may decrease at decay rate 168 or anotherlinear or step-wise decreasing manner. P-wave 256 crosses R-wave sensingthreshold 160 at time 259, starting a new blanking interval 172. Controlcircuit 80 determines RRI 267 as the time interval between the R-wavesensed event signals received at time 253 and time 259.

After the blanking interval 172 is started upon sensing P-wave 256 attime 259, the R-wave sensing threshold is set to a starting value 266based on a percentage of the maximum peak amplitude 258 of P-wave 256.The R-wave sensing threshold decreases at a decay rate 168 until itreaches a minimum sensing threshold value 170, e.g., the programmedsensitivity setting. R-wave 260 is sensed at time 261 when the cardiacelectrical signal 250 crosses R-wave sensing threshold 160. Controlcircuit 80 determines the next RRI 269 as the time interval betweenR-wave sensed event signals received from sensing circuit 86 at time 259and at time 261.

All of these RRIs 265, 267, and 269 are relatively short compared to atrue RRI 270 between consecutive R-waves 252 and 260. This cluster ofR-wave sensed events at times 251, 253, 259 and 261 defining relativelyshort RRIs may be identified and detected as evidence of PWOS by controlcircuit 80. Additional analysis of the sensed events, double sensing ofR-wave 252, P-wave 256 and R-wave 260 in this example, may be performedin response to detecting the cluster of sensed events, e.g., asdescribed in conjunction with FIG. 8.

FIG. 7 is a conceptual diagram of a bandpass filtered and rectifiedcardiac electrical signal 280 including R-waves 282 and 290 withintervening T-wave 284 and P-wave 286. In this example, the heart rateis very slow, e.g., less than 60 beats per minute or even slower at lessthan 40 beats per minute. R-wave 282 is properly sensed at time 295 whenthe cardiac electrical signal 280 crosses R-wave sensing threshold 160.After the blanking interval 172, R-wave sensing threshold 160 is set toa starting value 276 based on a percentage of the maximum peak amplitude285 of R-wave 282. R-wave sensing threshold 160 decreases at decay rate168 and is adjusted to the minimum sensing threshold value 170 after thepre-determined drop time interval 173. The drop time interval may be,for example, 1.5 seconds or 2 seconds in some examples. Because theheart rate is very slow in this example, P-wave 286 occurs after thedrop time interval 173 expires, and cardiac electrical signal 280crosses the R-wave sensing threshold 160 set to the minimum value 170.

P-wave 286 is sensed at time 297, starting a new blanking period 172.The R-wave sensing threshold 160 is adjusted to a starting value 288based on the peak amplitude of P-wave 286 and decays to the minimumthreshold value 170. R-wave 290 is sensed at time 299 when the cardiacelectrical signal 270 cross the R-wave sensing threshold 160. Controlcircuit 80 determines RRI 277 between R-wave sensed event signalsreceived from sensing circuit 86 at times 295 and 297 and determines RRI279 between R-wave sensed event signals received at times 297 and 299.These RRIs 277 and 279 are each shorter than the true RRI 292. The trueRRI 292 may be longer than a programmed lower pacing rate interval. Forexample, a lower pacing rate interval may be programmed to be 1.0seconds for a pacing rate of 60 pulses per minute or 1.5 seconds for apacing rate of 40 pulses per minute. If the RRI 277 is shorter than theprogrammed lower rate interval, a pacing escape interval started at time295, when R-wave 282 is sensed, may be reset at time 297 in response toan R-wave sensed event signal at time 297. PWOS may occur during a slowheart rate and lead to inhibition of pacing pulses when the true heartrate is slower than the rate corresponding to the programmed lower rateinterval.

Returning to FIG. 5A, sensed event clusters 104 may include multipleevents sensed at relatively short RRIs 106 due to PWOS. Each verticalline represents an event sensed as an R-wave by sensing circuit 86,resulting in an R-wave sensed event signal passed to control circuit 80.As such the intervals between the consecutively sensed events, such asinterval 106 and interval 108 are measured as RRIs by control circuit 80as the time interval between consecutively received R-wave sensed eventsignals. In this example, multiple P-waves are oversensed for eachR-wave due to an atrial tachyarrhythmia, with or without some degree ofatrioventricular conduction block. The degree of AV conduction block,(e.g., no AV conduction block, first degree, second degree, or thirddegree complete AV conduction block) affects how many P-waves areconducted to the ventricles and therefore affects how many P-waves occurduring one RR interval and the regularity of the P-R intervals during anatrial tachyarrhythmia. Each sensed event cluster 104 is separated fromthe next consecutive cluster 110 by a relatively long RRI 108. Controlcircuit 80 may apply PWOS detection criteria to R-wave sensed eventsignals and cardiac electrical signal waveforms occurring during sensedevent clusters to detect PWOS as described herein, e.g., in conjunctionwith FIGS. 8-14.

Depending on multiple factors, as illustrated by the examples of FIG.5C, FIG. 6 and FIG. 7, such as the intrinsic ventricular rate, R-waveamplitude, P-wave amplitude, underlying AV conduction, and intrinsicatrial rhythm (e.g., presence of an atrial arrhythmia such as atrialflutter, atrial fibrillation, atrial tachyarrhythmia, etc.), the numberof P-waves oversensed in each sensed event cluster 104, 110, etc., andthe relative RRIs during and between clusters, may vary. Clusterdetection criteria for detecting clusters of sensed events andidentifying PWOS may be established according to the cluster patternsobserved or expected in an individual patient based on their particularrhythm history and cardiac electrical signal characteristics. While FIG.5A depicts the RRI pattern during PWOS as multiple sensed event clustersseparated by relatively longer RRIs, in other cases PWOS may result inalternating long and short RRIs, e.g., in the example of FIG. 7 during aslow heart rate when a single P-wave 286 is oversensed during each trueRRI 292.

FIG. 8 is a flow chart 200 of a method for identifying and responding toPWOS according to one example. At block 202, control circuit 80 monitorsRRIs for clustered sensed events. One method for detecting a cluster ofsensed events is described in conjunction with FIG. 7. Briefly, RRIs aredetermined, e.g., based on the time or count that an escape intervaltimer has reached upon being reset due to the control circuit 80receiving an R-wave sensed event signal from sensing circuit 86. Longand short RRI thresholds may be applied to consecutively determined RRIsfor detecting a pattern of clustered sensed events. As shown in FIG. 5A,a pattern of clustered sensed events may include two or more short RRIs106 followed by a long RRI 108. In other examples, PWOS may becharacterized by S-L-S-L intervals when a single P-wave is oversensedduring each true RRI. This may occur during a slow heart rate, whenR-wave amplitude is low and AV conduction is intact. As such, in someexamples, a cluster of sensed events may include as few as two sensedevents separated by a short RRI from each other and separated from otherclusters of sensed events by one long RRI. In other examples, a clusterof sensed events may include 3, 4, 5 or more sensed events separated byshort RRIs. Multiple short RRIs within a cluster may vary from eachother but are all less than a short RRI threshold. The clustered sensedevents may include at least one P-wave and will typically include atleast one P-R or R-P interval as shown in the examples of FIGS. 5C, 6,and 7. The clustered sensed events may include a double-sensed R-wave,multiple oversensed P-waves, and oversensed T-waves in other examples.

If a cluster is detected based on cluster detection criteria, asdetermined at block 204, control circuit 80 may increase a clustercounter at block 206. The cluster counter may be compared to a thresholdat block 208. If the counter reaches a predetermined number of sensedevent clusters, e.g., 3, 4, 5 or other threshold number of clusters, thewaveforms of sensed events of each cluster are analyzed by controlcircuit 80 at block 210. Based on the waveform analysis, the cluster iseither identified or not identified as PWOS. The waveform analysis mayinclude event amplitude analysis, waveform morphology analysis, waveformslope analysis or other analysis of the event waveforms. One example ofthe analysis performed at block 210 is described below in conjunctionwith FIG. 8.

If PWOS is not identified, and all of the predetermined number ofclusters have not been identified, “no” branch of block 216, controlcircuit 80 analyzes the next cluster by returning to block 210. If thecluster is confirmed to be PWOS, “yes” branch of block 212, a PWOScounter may be increased at block 214 by control circuit 80. Once all ofthe predetermined number of clusters have been evaluated, “yes” branchof block 216, the PWOS count adjusted at block 214 is compared to a PWOSrejection threshold at block 218. If the PWOS counter does not reach arhythm rejection threshold, PWOS is not identified as indicated at block220. The heart rhythm being sensed or detected by the ICD is deemedvalid. For example, if VT or VF is being detected, the rhythm detectionis acceptable and a therapy may be delivered according to programmedtachyarrhythmia therapies. If VT or VF is not being detected and RRIsless than a programmed bradycardia lower pacing rate interval are beingdetermined, the RRIs are deemed correct. No bradycardia therapy isrequired.

If the number of clusters confirmed to be PWOS does reach the rejectionthreshold at block 218, PWOS is identified, and the currently sensedheart rhythm is rejected at block 222. If a VT or VF episode is beingdetected, the VT or VF detection may be rejected at block 222 or aventricular tachyarrhythmia therapy is withheld. If normal sinus rhythmis being sensed, e.g., if RRIs are being determined that are less than abradycardia lower pacing rate interval, with no bradycardia pacing beingdelivered, the sensed rhythm is rejected. ICD 14 may adjust therapycontrol parameters by enabling monitoring of event amplitudes, e.g., asdescribed in conjunction with FIG. 13, or other corrective actions maybe taken to identify oversensed P-waves as they occur so that they maybe ignored for the purposes of controlling ventricular pacing escapeinterval timers and ventricular pacing pulse delivery. Examples of aPWOS-based rhythm rejection response are described in conjunction withFIG. 11.

FIG. 9 is a flow chart 300 of a method that may be performed by ICD 14for detecting a sensed event cluster according to one example. Themethod of flow chart 300 may correspond to blocks 202, 204 and 206 ofFIG. 8 where RRIs are monitored for detecting a sensed event cluster anda cluster counter is increased when a cluster is detected. At block 302,control circuit 80 receives an R-wave sensed event signal from sensingcircuit 86. This sensed event signal is referred to as an “R-wave sensedevent signal,” because it is a signal produced in response to thecardiac electrical signal crossing the R-wave sensing threshold. Theactual event crossing the R-wave sensing threshold, however, maycorrespond to an R-wave, a P-wave, or even a T-wave or other non-cardiacelectrical noise, and thus may be a false R-wave sensed event signal.

At block 304, control circuit 80 determines RRI(N) as the time intervalending with the currently received R-wave sensed event signal andbeginning with the most recent preceding R-wave sensed event signal.This RRI(N) is compared to a short interval threshold (TH1) at block306. The short interval threshold TH1 may be set to a fixed interval,for example approximately 210 ms, 200 ms, 190 ms, 180 ms, 150 ms, orother predetermined time interval, which may be based on a normallyexpected P-R interval for the patient. The P-R interval, e.g., the timeinterval from a sensed P-wave or its maximum peak amplitude to the nextsensed R-wave or its maximum peak amplitude, may be determinedautomatically by ICD 14 or measured by a clinician. The short intervalthreshold TH1 may be automatically set or programmed to be slightlygreater than the expected P-R interval for the patient.

In some examples, RRI(N) (and subsequent RRIs) may be compared to aninterval range. When non-cardiac noise is present, such aselectromagnetic interference, very short RRIs may occur when thenon-cardiac noise is falsely sensed as R-waves. Accordingly, criteriafor identifying PWOS may include a minimum RRI threshold as well as amaximum RRI threshold for identifying short intervals between R-wavesensed event signals that may be caused by PWOS. The minimum RRIthreshold may correspond to a minimum expected P-R interval and themaximum RRI may correspond to a maximum expected P-R interval. A normalP-R interval range may be approximately 120 ms to 200 ms. This range maybe adjusted up or down or widened or narrowed based on individualpatient need.

If RRI(N) is not less than the short interval threshold, control circuit80 returns to block 302 to wait for the next R-wave sensed event signal.The RRI(N) is stored in memory 82, however, for use in evaluating aseries of RRIs. For example, a series of up to 3, 4, 6, or moreconsecutive RRIs needed for detecting a cluster pattern according tocluster detection criteria may be stored in memory 82 in a circulatingbuffer.

If RRI(N) is less than the short interval threshold at block 306, themost recent preceding RRI, RRI(N−1) may be compared to a long intervalthreshold (TH2) at block 308. The long interval threshold TH2 may be setto a minimum expected R-P interval when the heart rate is below atachyarrhythmia rate. For example, the long interval threshold TH2 maybe set to approximately 250 ms, 300 ms, 350 ms, 400 ms or otherpredetermined time interval. A short RRI preceded by a long RRI may bethe onset of a cluster of sensed events. If the preceding interval isnot longer than the long interval threshold, the control circuit 80waits for the next R-wave sensed event signal at block 302. If thepreceding RRI(N−1) is longer than the long interval threshold, theevents defining the beginning and end of RRI(N) may be clustered events.A cluster interval counter is set to one at block 310 to begin countingthe number of RRIs following the longer RRI(N−1) that are less than theshort interval threshold.

At block 312, control circuit 80 determines the next RRI, RRI(N+i) wherei is initially set to 1 at block 311. The next RRI(N+1) is compared tothe short interval threshold at block 314. If the comparison at block314 is true, the cluster interval counter is again increased at block316. The cluster interval counter is compared to a maximum number ofshort intervals at block 318. If a maximum number of intervals has notbeen reached, but the cluster interval counter has been increased, acluster is being detected at bock 320. In this example, if at least twoconsecutive short intervals follow a long interval based on the shortand long interval thresholds or ranges, a sensed event cluster is beingdetected. In other examples, the cluster interval counter may becompared to a cluster detection threshold that requires a predeterminednumber of short intervals in order to detect a cluster. In some cases,as few as one short interval (preceded and followed by a long interval)may be detected as a cluster of sensed events. In other examples, atleast two or at least three short intervals may be required to detect asensed event cluster such that at least three sensed events or at leastfour sensed events are clustered together at short intervals.

The cluster interval counter is compared to a maximum limit at block 318so that events occurring at a sustained fast rate are not detected as avery long cluster, leading to PWOS detection. Events occurring at asustained fast rate may be a true ventricular tachyarrhythmia. When thecluster interval counter exceeds a maximum number of short intervals,therefore, the cluster detection may be cancelled at block 328. Thecluster interval counter is cleared at block 326 and the control circuit80 returns to block 302 to repeat the process beginning with the nextR-wave.

If a cluster is being detected (block 320), i is increased by one atblock 322, where i is used to identify the next RRI(N+i) interval. TheRRI(N+i) is determined at block 312 and compared to the short intervalthreshold at block 314. If RRI(N+i) is not less than the short intervalthreshold, and a cluster is being detected based on one or morepreceding short RRIs, as determined at block 324, a cluster counter isincreased by one at block 330. If a cluster is not being detected, forexample if the cluster interval counter has not been increased arequired number of times based on comparisons of the RRIs to the shortinterval threshold or range, the cluster interval counter is cleared atblock 326. A sensed event cluster is not detected and the clusterinterval counter is not increased. Control circuit 80 returns to block302 to continue monitoring RRIs.

If the cluster counter is increased at block 330, the cluster countermay be compared to a counter threshold at block 208 of flow chart 200(FIG. 6) as described previously. Once a threshold number of sensedevent clusters are identified based on RRIs, the clustered eventwaveforms may be analyzed at block 210 for identifying PWOS. In someexamples, as few as one cluster of two sensed events occurring at oneshort RRI preceded and followed by long RRIs, based on the long andshort RRI thresholds or ranges, may result in a cluster being detectedand the cluster counter reaching a counter threshold at block 208 tocause control circuit 80 to perform further analysis for identifyingPWOS at block 210.

FIG. 8 is a flow chart 400 of one method for analyzing the clusteredevent waveforms at block 210 of FIG. 6 for identifying a sensed eventcluster as PWOS at block 212. After the cluster counter reaches athreshold number of clusters, e.g., 1, 2, 3, 4, 5 or other predeterminednumber of clusters, analysis of the event waveforms of each clusterbegins at block 402. The control circuit 80 may receive a digitized,filtered and rectified cardiac electrical from sensing circuit 86 thatis buffered in memory 82 in a circulating buffer to be available forwaveform analysis if the cluster counter reaches the threshold.

Each cluster may be defined to start on the first R-wave sensed eventsignal defining the beginning of the first short RRI(N) that is lessthan the short interval threshold (and defines the end of theimmediately preceding long RRI(N−1) that is greater than the longinterval threshold). The sensed event cluster may also be defined to endon the last R-wave sensed event signal that defines the first RRI(N+i)that is not less than the short interval threshold after the clusterinterval counter begins counting short RRIs. Alternatively, the sensedevent cluster may end on the preceding sensed event that defines thebeginning of the last short RRI. The last R-wave that defines the end ofthe last short RRI and the start of the first long RRI after one or moreshort RRIs may be an R-wave as shown in FIG. 5A.

In one example, the maximum peak amplitude for each sensed event of agiven cluster, including the starting and ending events, may bedetermined during a blanking interval following an R-wave sensingthreshold crossing. The maximum peak amplitude may be stored in memory82 in a circulating buffer with the timing of the corresponding R-wavesensed event signal. When the cluster counter reaches the threshold,control circuit 80 may determine the largest maximum peak amplitudestored for each respective R-wave sensed event during a given cluster atblock 402. Alternatively the largest maximum peak amplitude out of allevents during the cluster may be determined from the buffered, digitizedcardiac electrical signal.

The largest maximum peak amplitude for the sensed event cluster iscompared to an amplitude threshold at block 404. In some patients, PWOSmay be most likely to occur when a relatively small amplitude R-waveoccurs causing the R-wave sensing threshold starting value to be set toa relatively low amplitude as illustrated in FIG. 5C. As such, onecriterion for identifying a cluster of sensed events as PWOS may be thatthe largest maximum peak amplitude be less than a threshold amplitude,indicating that if the cluster of events includes a true R-wave, it is asmall amplitude R-wave and otherwise all of the sensed events have anamplitude that is not greater than an expected P-wave amplitude. Theamplitude threshold may be approximately 200 millivolts or set based ondetermining a baseline P-wave and R-wave amplitude.

Amplitude criteria for identifying PWOS may require that the largestmaximum peak amplitude of all sensed events of a cluster be less thanthe amplitude threshold at block 404. In other examples, a percentage orportion of all sensed events of a cluster may be required to be lessthan the amplitude threshold. For example, if a cluster includes foursensed events, three out of the four events may be required to have amaximum amplitude less than the amplitude threshold. A normal R-wave maybe sensed during a sequence of oversensed P-waves as shown in FIG. 5A,e.g., when an atrial arrhythmia is occurring and the ventricular rate isslow and/or R-wave amplitude to P-wave amplitude ratio is relativelysmall. As such, amplitude criteria for detecting PWOS may allow for oneor more large amplitude sensed events to occur as long as apredetermined percentage or portion of the sensed event waveforms have amaximum amplitude that is less than the amplitude threshold.

In other examples, the last sensed event of an event cluster may beexcluded from comparison to an amplitude threshold at block 404. Arelatively large amplitude R-wave may reset the starting R-wave sensingthreshold value to a relatively high value that precludes PWOS on thenext beat. The last sensed event of an event cluster that starts the RRIthat is greater than the short interval threshold, concluding the seriesof short RRIs, may be a true R-wave. As long as all, or at least apredetermined portion, of the sensed events prior to the last sensedevent of the cluster are less than the amplitude threshold, the clusterof sensed events may satisfy amplitude criteria required for identifyingPWOS at block 404.

If the amplitude criteria for identifying PWOS are not satisfied atblock 404, the cluster is not identified as PWOS at block 412. If theamplitude criteria are met, additional waveform morphology analysis maybe performed at block 406. In one example, a morphology matching scoreis determined for each sensed event waveform of the cluster by comparingthe waveform to a known R-wave template. An R-wave template may bepreviously generated and stored in memory 82, e.g., by aligning andaveraging multiple R-wave signals acquired during normal sinus rhythm.The waveform of each sensed event may be aligned with the R-wavetemplate and the differences between each aligned pair of sample pointsmay be determined for determining a morphology match score for eachwaveform. Various morphology matching algorithms may be used, includingwavelet transform or other transform methods. Examples of methods forgenerating an R-wave template and determining a morphology matchingscore are generally disclosed in U.S. Pat. No. 6,393,316 (Gillberg, etal.), U.S. Pat. No. 8,825,145 (Zhang et al.), U.S. Pat. No. 8,965,505(Charlton, et al.), and U.S. Pat. No. 8,983,586 (Zhang et al.), all ofwhich are incorporated herein by reference in their entirety.

At block 408, control circuit 80 determines if morphology criteria foridentifying PWOS are met. In one example, the morphology matching scoresfor each event of the cluster determined at block 406 are compared to amatch threshold. As long as all or a predetermined percentage or numberof all of the sensed event waveforms of the cluster have a matchingscore that is less than the match threshold, the PWOS morphologycriteria are met at block 408. A low matching score indicates arelatively poor correlation between the sensed event waveforms and theR-wave template, indicating that the waveforms sensed as R-waves areunlikely to be true R-waves. For example, if the morphology matchingscore has a possible value of between 0 and 100, a morphology matchingscore of 30 or less may indicate that the event is highly unlikely to bea true R-wave. In some cases, at least one true R-wave is expected tooccur within an event cluster, e.g., as shown in FIG. 5A. In this case,at least one event in a detected cluster of sensed events may berequired to have a morphology matching score greater than a matchthreshold, e.g., greater than 30, and all remaining events, which may beone or more, may be required to have morphology match score less thanthe threshold.

If the PWOS morphology criteria are not met at block 408, the sensedevent cluster is not identified as PWOS. The cluster of sensed eventsmay be a true arrhythmia or may be caused by other cardiac oversensing,e.g., T-wave oversensing, or other non-cardiac oversensing such asoversensing of electromagnetic interference, muscle noise, or othernon-cardiac noise. If the PWOS morphology criteria are met at block 408,the sensed event cluster is identified as PWOS at block 410. The processof FIG. 10 may be repeated for each of the clusters that were countedtoward reaching the cluster threshold. In other examples, clusters ofsensed events are identified going forward in time after identifying onecluster as PWOS. Each cluster identified as being PWOS is counted atblock 214 of FIG. 8 and if the PWOS counter reaches a threshold numberof PWOS-identified clusters, a response to identifying the PWOS isprovided as described above in conjunction with FIG. 8 and as describedin conjunction with FIG. 11 below.

FIG. 11 is a flow chart 500 of a method performed by ICD 14 foridentifying and responding to PWOS according to one example. At block502, bradycardia PWOS criteria are established and stored in memory 82for access by control circuit 80 in detecting sensed event clusters andidentifying PWOS during a bradycardia heart rhythm. At block 504,tachyarrhythmia PWOS criteria are established and stored for detectingsensed event clusters and identifying PWOS during tachyarrhythmia. PWOSmay present a different pattern of sensed event clusters when the heartrhythm is slow, during bradycardia, than when the heart rhythm is fast,during tachycardia or fibrillation.

For example, the sensed event clusters during bradycardia may include asingle short RRI when the sensed event pattern is an oversensed P-wavealternating with a sensed R-wave in a P-R-P-R pattern, which may persistover many cardiac cycles (multiple, sequential 2-event clusters) or overa few cardiac cycles with a series of intervening cardiac cycles withcorrectly sensed R-waves at normal RRIs and no oversensing. During afast sensed heart rate, multiple P-waves may be oversensed for each trueR-wave with none, one or more true R-waves sensed between sensed eventclusters. Accordingly two or more different sets of PWOS detectioncriteria may be defined including different short and/or long intervalthresholds used to detect clustered events, the number of shortintervals required to detect a cluster, the number of sensed eventclusters required to be identified before analyzing sensed eventwaveforms, the amplitude and/or morphology criteria used to identifyPWOS in a sensed event cluster, and/or the number of clusters identifiedas PWOS required to provide a response to PWOS. Different criteria maybe defined for detecting PWOS in the presence of bradycardia, in thepresence of tachyarrhythmia, in the presence of AV conduction block orother conditions the patient may be known or expected to experience andwhich may influence the patterns of sensed events during PWOS.

After establishing at least two different sets of criteria, which mayinclude establishing thresholds, an R-wave template and other criteriabased on an analysis of the patient's cardiac electrical signal, controlcircuit 80 monitors for sensed event clusters according to the two (ormore) sets of PWOS criteria. Sensed event signals and the cardiacelectrical signal waveforms may be monitored for sensed event clustersaccording to multiple PWOS criteria simultaneously in order to detectdifferent patterns of PWOS. Alternatively, the sensed event rate may beused to determine which set of PWOS criteria is actively being used. Forexample, if the sensed event rate over a predetermined number of sensedevents, such as the most recent, 8, 12, 18, 22, or other number ofsensed events is greater than a tachyarrhythmia detection rate, then thePWOS criteria for tachyarrhythmia established at block 504 are used. Forexample, if a running average RRI is less than 500 ms, less than atachycardia detection interval, or if a tachyarrhythmia intervaldetection counter is greater than a predetermined number such as 3, thetachyarrhythmia PWOS criteria is used. If the heart rate is less than120 beats per minute or a running average RRI is longer than thetachycardia detection interval and/or a tachyarrhythmia intervaldetection counter is inactive (at a count of zero), the bradycardia PWOScriteria may be used.

If PWOS is detected according to the bradycardia PWOS criteria, “yes”branch of block 510, one or more PWOS responses may be provided at block512, 514 and/or 516. Bradycardia pacing may be enabled at block 512 inresponse to identifying PWOS based on the bradycardia PWOS criteria. Atriggered pacing mode, e.g., VVT pacing mode, may be started to enableventricular pacing at a rate that is faster than the intrinsic heartrate and is triggered from the next sensed event, which may be P-wave oran R-wave. A triggered pacing pulse is delivered by the therapy deliverycircuit within a physiological refractory period of a sensed event,e.g., within 100 ms or within no more than 200 ms of the sensed eventwithout setting a pacing escape interval upon sensing the event.

An inhibited pacing mode, e.g., VVI pacing mode, may be started at block512 during which a sensed event signal inhibits a scheduled pacing pulseonly if the sensed event is confirmed to be an R-wave. In an inhibitedpacing mode, a pacing escape interval is started upon each sensed event(that occurs outside any device blanking or refractory periods). Apacing pulse is scheduled to be delivered if the pacing escape intervalexpires without being restarted due to another sensed event. Uponidentifying PWOS and enabling bradycardia pacing, R-wave sensed eventconfirmation may be enabled at block 512 as part of the bradycardiapacing control. Before restarting the pacing escape interval in responseto a sensed event, one or more morphology features may be determined forconfirming each sensed event as being an R-wave. For example, the peakamplitude, positive going slope, morphology matching score or othermorphological feature of each sensed event may be determined andcompared to R-wave confirmation criteria. If the sensed event isconfirmed to be an R-wave based on the confirmation criteria beingsatisfied, a running pacing escape interval is restarted in response tothe confirmed R-wave so that the pacing pulse scheduled at theexpiration of the escape interval is withheld. If the sensed event isnot confirmed to be an R-wave, a running pacing escape interval is notrestarted but is allowed to continue running until either a sensed eventis confirmed to be an R-wave and the pacing escape interval is restartedor the pacing escape interval expires and the scheduled pacing pulse isdelivered, whichever occurs first.

Alternatively or additionally, one or more parameters used to controlthe R-wave sensing threshold may be adjusted at block 514. For example,the R-wave sensing threshold starting value may be increased, a decayrate may be decreased, a time interval at which the sensing threshold isdropped to a lower value may be increased, the minimum sensing thresholdmay be increased, or other parameter may be adjusted to effectivelyincrease the R-wave sensing threshold at the time the P-wave is expectedto avoid PWOS on subsequent heart beats.

If PWOS is not being identified according to the bradycardia PWOScriteria but is being identified according to the tachyarrhythmia PWOScriteria, “yes” branch of block 518, ICD 14 may provide one or moreresponses to identifying the PWOS. For example if a tachyarrhythmiaepisode detection is in progress, as determined at block 520, detectionof the tachyarrhythmia episode may be withheld and/or a VT or VF therapymay be withheld at block 522. A tachyarrhythmia episode detection may bedetermined to be in progress at block 520 if at least onetachyarrhythmia detection interval counter, e.g., a counter used tocount the number of RRIs falling into a tachycardia interval zone or acounter used to count the number of RRIs falling into a fibrillationinterval zone, is active, e.g., has a count that is greater than zero oranother predetermined count.

Any time that PWOS is identified, control circuit 80 may respond atblock 514 by adjusting R-wave sensing threshold control parameters toreduce the likelihood of oversensing of P-waves in the future. At block516, ICD 14 may response to PWOS anytime it is identified by storingdata relating to the identified PWOS in memory 82 for transmission to anexternal device 40 (FIG. 1) via telemetry circuit 88. Stored data mayinclude a cardiac electrical signal including one or more sensed eventclusters identified as PWOS, RRI data, morphology data, subsequenttherapy delivered or withheld, or the like. Storage and transmission ofdata pertaining to the identified PWOS may enable a clinician to adjustprogrammed sensing parameters, PWOS detection criteria, tachyarrhythmiadetection criteria, bradycardia pacing control parameters and/ortachyarrhythmia therapy control parameters to optimize the performanceof ICD 14 in reliably determining the heart rhythm and delivering orwithholding therapy as needed.

FIG. 12 is a flow chart of a method for identifying PWOS by ICD 14according to another example. At block 602, sensed event signalsproduced by sensing circuit 86 are received by control circuit 80. Asthe sensed event signals are received, the maximum peak amplitude ofeach sensed event is determined at block 604. In some examples, ablanking interval is started at the time that the cardiac electricalsignal crosses the R-wave sensing threshold, such as blanking interval172 shown in FIG. 5B. The control circuit 80 may determine the maximumsample point amplitude during the blanking interval 172 as the maximumpeak amplitude of the respective sensed event.

At block 606, control circuit 80 determines a morphology matching scorefor each sensed event according to an implemented morphology matchingscheme, such as any of the examples provided above and in theincorporated references. The morphology matching score is determined foreach sensed event by comparing the waveform morphology or one or moremorphology features of the cardiac signal waveform corresponding to therespective sensed event to a predetermined normal R-wave morphologytemplate or normal R-wave features.

At block 608, the sensed event interval is determined as the timeinterval between two consecutive sensed event signals received at block602. It is recognized that when the process of flow chart 600 firstbegins, a sensed event interval ending on the very first event sensed bysensing circuit 86 will not be determined since a most recent precedingsensed event will not exist. The very first event sensed by ICD 14 maybe used, therefore, to set an initial time marker of the first sensedevent to enable determination of the first sensed event interval endingwith the second sensed event signal at block 608. The sensed eventintervals determined at block 608 may be referred to herein as “RRIs”since they are determined based on R-wave sensed event signals, but thesensed event intervals may not be true “RRIs” since one (or both) of thesensed events defining the beginning and end of a sensed event intervalmay not be a true R-wave. For example, one or both sensed events may bean oversensed P-wave or other oversensed event.

These parameters (event amplitude, event morphology and event interval)may be determined at blocks 604, 606 and 608 for each sensed event as itoccurs and stored in memory 82 at block 610. These three parametersdetermined for each sensed event may be used to determine if the sensedevents are likely to include PWOS based on determined patterns of thesuccessive event amplitudes, event morphologies and sensed eventintervals.

At block 612, the control circuit 80 performs a comparative analysis ofthe sensed event parameters. In some examples, the event parameters foreach event are first compared to a predetermined amplitude threshold, apredetermined morphology match threshold and a predetermined RRIthreshold. In other examples, the analogous event parameters determinedfor each sensed event may be compared to each other. The comparativeanalysis is performed to determine if parameter values of consecutivelysensed events represent a likely PWOS pattern. For instance, thecomparative analysis may be performed to determine if consecutivelysensed events includes groups of events that present an alternatingpattern of low and high amplitude, R-wave and non-R-wave morphology,and/or short and long RRIs, which would be evidence of an alternatingpattern of oversensed P-waves and true R-waves.

In one example, at block 612, the maximum peak amplitude determined foreach sensed event may be compared to a maximum P-wave amplitudethreshold, e.g., 1.5 mV. The maximum P-wave amplitude threshold may be apredetermined value based on empirical data or selected for the patientbased on actual P-wave peak amplitude measurements (and/or R-wave peakamplitude measurements). If the event amplitude is less than the maximumP-wave amplitude threshold, the event may be labeled or flagged as a lowamplitude event, “L.” If the event amplitude is greater than the maximumP-wave amplitude threshold, the event may be labeled or flagged as ahigh amplitude event or “H.”

Each sensed event interval may be compared to an RRI threshold at block612. The RRI threshold may be a predetermined minimum event intervalthat would be considered a valid RRI when a tachyarrhythmia is notoccurring, e.g., 300 ms. If the sensed event interval is less than theRRI threshold, the sensed event interval may be labeled as short or “S”and otherwise labeled as long or “L.”

Additionally or alternatively at block 612, the morphology matchingscore for each event may be compared to an R-wave matching threshold.For example, the R-wave matching threshold may be 30 when the maximumpossible matching score is 100. If the morphology matching score is lessthan the R-wave matching threshold, the event may be flagged or labeledas having a non-matching morphology, or “N,” indicating that the sensedevent is not likely to be an R-wave. If the morphology matching score isgreater than 30 the event may be labeled or flagged as having a matchingmorphology, “M,” indicating that the sensed event may be a true R-wave.Once at least three consecutive sensed events are labeled according tothe three event parameters of amplitude, morphology and sensed eventinterval at which the event occurs, the three labels may be compared todetermine if an event pattern indicative of PWOS is presented at block614.

FIG. 14 is a timing diagram 700 of sensed event signals 702 that may beproduced by sensing circuit 86 and received by control circuit 80. Thesensed event signals 702 arrive in multiple sensed event clusters 711,712, 713, 714, 715 and 716. Each cluster includes at least oneoversensed P-wave (e.g., cluster 713) or multiple oversensed P-waves(e.g., cluster 714).

In response to each sensed event, control circuit 80 analyzes thedigitized cardiac electrical signal and the time interval betweenconsecutive sensed event signals to determine the maximum absolute peakamplitude of each event, a morphology matching score, and a sensed eventinterval as described above in conjunction with FIG. 12. Control circuit80 may then label each event as having a high (H) or low (L) amplitude704, having a non-matching (N) or matching (M) morphology matching score(MMS) 706, and as the ending event of a short (S) or long (L) RRI 708.This event parameter labeling may be based on comparisons topredetermined thresholds as described above in conjunction with FIG. 12.

Alternatively, the event labels may be determined based on comparing theevent parameters to each other. For example, sensed event signals may beanalyzed in pairs of two consecutive events, three consecutive events orgroups of more than three consecutive sensed events, such as the groupof four consecutive sensed events 720. The event having the highestamplitude within the group of consecutive sensed events may beidentified and the amplitudes of each of the other events may becompared to the highest amplitude. The other event amplitudes that areless than a predetermined percentage of the highest amplitude arelabeled as “L” for low amplitude. The other event amplitudes that aregreater than a predetermined percentage of the highest event amplitudeare labeled as “H” for high amplitude.

The highest morphology matching score may be identified and compared tothe morphology matching scores of each of the other sensed events of theselected group of consecutive events, e.g., group 720. If the othermorphology matching scores are greater than a predetermined percentageof the highest morphology matching score, the corresponding events arelabeled “M” to indicate a morphology that is likely an R-wave. Anyevents of the selected group having morphology matching scores that areless than the predetermined percentage of the highest morphologymatching score are labeled as non-matching or “N.”

The longest RRI of the selected group of sensed events may be identifiedand compared to the other RRIs of the selected group. Events ending anRRI that is at least a predetermined percentage of the longest RRI arelabeled as “L” (long). Events ending an RRI that is less than thepredetermined percentage of the longest RRI are labeled as “S” (short).

In some examples, the sensed events may be labeled based on acombination of comparisons of event parameters to a predeterminedthreshold and comparisons to each other. For example, the RRIs may becompared to a predetermined threshold for labeling an event as occurringat a long or short interval. The highest morphology matching score maybe determined and, as long as it is greater than a predeterminedthreshold, the corresponding event may be labeled as M (matching anR-wave morphology). Other event morphology matching scores may becompared to the same predetermined threshold or to a percentage of thehighest morphology matching score. The maximum peak amplitude of thesensed event having the highest morphology matching score may bedetermined and the maximum P-wave sensing threshold may be set as apercentage of the maximum peak amplitude. The maximum peak amplitudes ofthe other sensed events of the selected group of events may be comparedto the maximum P-wave sensing threshold determined based on the maximumamplitude of the event having the highest morphology matching score.Events having a peak amplitude less than the P-wave sensing thresholdare labeled as “L” and event having a peak amplitude greater than theP-wave sensing threshold are labeled as “H.” It is understood that othervariations or combinations of comparisons of the event parameters toeach other and/or to predetermined thresholds may be conceived andutilized for labeling the sensed events as being relatively low or highin amplitude, having a relatively low R-wave morphology matching scoreor a relatively high R-wave morphology matching score, and/or ending ona relatively short RRI or a relatively long RRI. The event labelingallows control module 80 to determine if a PWOS pattern is present.

Event labels for the parameters of amplitude 704, morphology matchingscore (MMS) 706 and RRI 708 are shown in timing diagram 700 for eachsensed event signal 702. Control circuit 80 may analyze the sensed eventlabels for amplitude 704, MMS 706 and RRI 708 in running groups of foursensed events for identifying alternating patterns of L-H-L amplitude,N-M-N morphology matching scores, and S-L-S RRIs. For example, the firstgroup of four sensed events 720 for which each of the parameters ofamplitude, MMS and RRI are determined is selected for pattern analysis.An RRI is not determined for the very first sensed event signal 701. Assuch, a group of four consecutive sensed events 720 allows for the firstthree amplitude and morphology labels of the group of four sensed eventsto be examined and the last three event interval labels to be examinedfor an alternating PWOS pattern.

The amplitude labels and MMS labels for the first three sensed eventsignals of the first group of four sensed events 720 are shown in dashedboxes 705 and 707, respectively. The pattern of the amplitude and MMSlabels are analyzed to determine if the first three sensed events of thegroup of four sensed events 720 occur in an alternating L-H-L and N-M-Npattern, respectively.

The RRI labels of the last three sensed events of the group of foursensed events 720 are shown in dashed box 709. The RRI labels of thelast three sensed events of the group of four sensed events 720correspond to the three respective RRIs that begin with the first threesensed events of the group of four sensed events 720. Control circuit 80analyzes the RRI labels of the last three sensed events of the group offour sensed events 720 for an alternating pattern of S-L-S.

As can be seen in the example of FIG. 14, the first group of four sensedevents 720 satisfies the criteria of an L-H-L amplitude pattern of thefirst three sensed events, an N-M-N morphology matching score pattern ofthe first three sensed events, and an S-L-S pattern of the RRI labels ofthe last three sensed events. As such, this group of four sensed events720 is identified by the control circuit 80 as evidence of sensed eventcluster 711 (and 712). In this example, the evidence of the L-H-Lamplitude pattern, N-M-N morphology pattern, and S-L-S RRI patterncorresponds to the last short interval of sensed event cluster 711, thefirst short interval of sensed event cluster 712, and the interveninglong sensed event interval 703 separating the two sensed event clusters711 and 712. In response to identifying evidence of sensed eventclusters 711 and 712, a PWOS cluster interval counter included incontrol circuit 80 may be increased by one. When the count of the PWOScluster interval counter reaches a threshold, the control circuit 80 mayprovide a PWOS response, e.g., any of the responses described above inconjunction with FIGS. 8 and 11 or below in conjunction with FIG. 13.

After determining whether the first group of four sensed event intervals720 present a pattern of PWOS as evidence of sensed event clusters,control module 80 may advance by one sensed event signal to the nextgroup of four sensed events to analyze the next group of fourconsecutive sensed events, which in this example present H-L-L amplitudepattern, M-N-N morphology matching score pattern, and L-S-S RRI pattern.The next group of four sensed events is appropriately not identified asevidence of a new sensed event cluster since these events are still partof the sensed event clusters 711 and/or 712. In some examples, when agroup of four sensed events is identified as evidence of sensed eventclusters, the control circuit 80 may advance by two sensed events ratherthan only one sensed event to select the next group of four sensedevents for analysis for a PWOS pattern. Advancement by only one sensedevent may result in selecting four sensed events that still occur withinthe same two sensed event clusters that were just identified.

This process of selecting groups of four consecutive sensed events andanalyzing the amplitude, MMS and RRI labels may continue until athreshold number of the groups of four sensed events are identified asbeing evidence of sensed event clusters. In the example shown, thegroups of four sensed events 720, 722, 724, 726 and 728 are eachidentified as evidence of sensed event clusters 711 through 716 based onthe L-H-L amplitude pattern, the N-M-N MMS pattern, and the S-L-S RRIpattern (each highlighted by respective dashed boxes for each group offour sensed events 720 through 728).

In each case, the sensed event clusters 701 through 716 are identifiedbased on the last short interval of one cluster, the first shortinterval of the immediately following cluster, and the intervening longRRI. This analysis by control circuit 80 identifies the sensed eventclusters 711 through 716 independent of the number of oversensed eventsand short RRIs occurring within each cluster 711 through 716. When thenumber of oversensed events and short RRIs within each cluster isvariable, as shown in FIG. 14, the presence of the sensed event clustersis still identified without having to detect a threshold number of shortRRIs within a cluster. The PWOS pattern may vary between patients andwithin a given patient as shown by the various examples of FIGS. 5B, 5C,6 and 7. The techniques represented by FIG. 12 and FIG. 14 can besuccessfully implemented for identifying sensed event clusters due toPWOS without requiring prior knowledge of an anticipated or predictednumber of events within the event clusters when PWOS occurs.

Returning to FIG. 12, if the PWOS pattern criteria are not met by thesensed event parameters based on the comparisons made at block 612, “no”branch of block 614, the control circuit 80 advances to block 616 todetermine and store the next set of sensed event parameters uponreceiving the next sensed event signal from sensing circuit 86.

If the comparisons of the sensed event parameters made at block 612 domeet PWOS pattern criteria, “yes” branch of block 614, a sensed eventcluster is detected at block 618. The PWOS pattern criteria may requirean alternating pattern of all three of the sensed event parametersdetermined for groups of consecutive events as described in conjunctionwith FIG. 14. In some cases, two out of the three sensed eventparameters may be required to present an alternating pattern in order tosatisfy the PWOS pattern criteria at block 614. For example, if thegroup of four consecutively sensed events present at least two of anL-H-L amplitude pattern, an N-M-N morphology matching score pattern, andan S-L-S RRI pattern, the PWOS pattern criteria may be satisfied atblock 614. In some cases, as long as the S-L-S RRI pattern is detected,either an L-H-L amplitude pattern or an N-M-N morphology matching scorepattern (or both) will satisfy the PWOS pattern criteria at block 614.In still other examples, an alternating patter of only one of the threesensed event parameters within the group of four consecutively sensedevents may satisfy the PWOS pattern criteria at block 614.

In the example of FIG. 12, if the PWOS pattern criteria are met a singletime, a sensed event cluster is detected at block 618, and the controlcircuit 80 enables R-wave confirmation at block 620 for use incontrolling therapy delivery. In other examples, a threshold number ofsensed event cluster detections may be required based on the PWOSpattern criteria being met multiple times before enabling R-waveconfirmation at block 620.

FIG. 13 is a continuation of flow chart 600. If R-wave confirmation isenabled at block 620 of FIG. 12, control circuit 80 advances to block652 of FIG. 14 (as indicated by connector “B”) to wait for the nextsensed event signal at block 652. Control circuit 80 switches frommonitoring for PWOS as described in conjunction with FIG. 12 and FIG. 14to performing R-wave confirmation as shown by the method of FIG. 13 toidentify PWOS events on a beat-to-beat basis and reduce the likelihoodof an improper withholding or delivery of therapy due to PWOS. Asindicated by block 651, the pacer timing and control circuitry ofcontrol circuit 80 may be running an escape interval timer or counterthat was started upon the immediately preceding sensed event signal. Asdescribed above in conjunction with FIG. 4, the escape interval timermay be started for timing out a pacing time interval for controlling thetiming of pacing pulse delivery. The escape interval timer may berestarted in response to a sensed event signal from sensing circuit 86and the value of the timer may be used for determining the sensed eventinterval since the immediately preceding sensed event. The sensed eventinterval may be used both for PWOS detection as described in conjunctionwith FIGS. 12 and 14 and for detecting tachyarrhythmia, e.g., accordingto VT and VF detection algorithms.

At block 654, control circuit 80 determines the maximum peak amplitudeof the currently sensed event, sensed t block 652 after R-waveconfirmation is enabled. The peak amplitude is compared to a P-waveamplitude threshold at block 656. If the peak amplitude of the sensedevent is less than the P-wave amplitude threshold, “yes” branch of block656, control circuit 80 may identify the sensed event as being a PWOSevent at block 658 or at least does not confirm the sensed event asbeing an R-wave. Resetting of the running escape interval is withheld atblock 660. The currently running escape interval timer is allowed tocontinue running at block 651 without being reset due to the sensedevent signal. In this way, the PWOS event does not interfere with thescheduling and timing of pacing pulses or the detection of atachyarrhythmia.

For instance, a bradycardia pacing escape interval may be started bycontrol circuit 80 in response to the most recent preceding sensed eventsignal. This running escape interval is allowed to continue to runwithout being reset so that the PWOS event does not prevent bradycardiapacing when it is needed. The PWOS does not cause the escape interval tobe reset which may otherwise lead to a false RRI determination that maybe less than a pacing interval and lead to withholding of a pacingpulse, or the false RRI may be in a tachycardia or fibrillation intervalrange and lead toward a false tachyarrhythmia detection.

If the peak amplitude is greater than the P-wave amplitude threshold,“no” branch of block 656, the sensed event is confirmed to be an R-waveat block 662. A pacing escape interval timer is reset at block 664 inresponse to the confirmed R-wave. It is to be understood that when theescape interval timer is reset, the time expired on the escape intervaltimer is used as a determination of the RRI ending on the confirmedR-wave, and this RRI may be used by tachyarrhythmia detection algorithmsimplemented in ICD 14. It is also to be understood that if the escapeinterval expires before a confirmed R-wave causes the escape interval tobe restarted, a pacing pulse may be delivered by ICD 14.

In some examples, control circuit 80 may determine at block 666 if noPWOS events have been identified over a predetermined maximum timeinterval. If PWOS has not been identified for a predetermined maximumtime interval, e.g., for one minute, 5 minutes, one hour, 24 hours, orother desired interval, control circuit switches back to the PWOSmonitoring mode by returning to block 602 of FIG. 12 (as indicated byconnector “A”). In this way, a beat-by-beat confirmation of R-waves isnot performed if PWOS is not detected for the predetermined maximum timeinterval set for controlling how long R-wave confirmation takes place ona beat-by-beat basis in the absence of PWOS detection.

Alternatively, if PWOS is detected at least once, before or afterenabling R-wave confirmation, the ICD 14 may remain in the monitoringmode in which all sensed events are analyzed to confirm whether or notthe event is a true R-wave until ICD 14 is reprogrammed by a user. Insome examples, the R-wave confirmation mode described in conjunctionwith FIG. 13 may be entered directly at block 652 without requiring PWOSmonitoring to be performed first as described in conjunction with FIG.12.

Enabling the PWOS monitoring mode only (FIG. 12), enabling the R-waveconfirmation mode only (FIG. 13) and enabling the PWOS monitoring modewith automatic switching to the R-wave confirmation mode (combination ofFIGS. 12 and 13) may be user-programmable features of ICD 14. When thePWOS monitoring mode as described in conjunction with FIGS. 12 and 14 isenabled without automatic switching to the R-wave confirmation mode inresponse to one or a higher predetermined number of sensed eventclusters being identified, the ICD 14 may respond to a predeterminednumber of sensed event clusters being identified during the monitoringmode by recording a cardiac electrical signal segment includingidentified PWOS, adjusting R-wave sensing threshold control parameters,withholding a tachyarrhythmia episode detection and/or therapy,delivering one or more triggered pacing pulses within the physiologicalrefractory period of one or more respective sensed events to providebradycardia pacing support if needed, or any combination of theseresponses or other PWOS responses described herein.

Thus, a method and apparatus for identifying and responding to PWOS inan extra-cardiovascular ICD system have been presented in the foregoingdescription with reference to specific embodiments. In other examples,various methods described herein may include steps performed in adifferent order or combination than the illustrative examples shown anddescribed herein. It is appreciated that various modifications to thereferenced embodiments may be made without departing from the scope ofthe disclosure and the following claims.

The invention claimed is:
 1. An implantable cardioverter defibrillator(ICD) system, comprising: a sensing circuit configured to: receive acardiac electrical signal from electrodes coupled to the ICD, and sensea plurality of consecutive cardiac events from the cardiac electricalsignal, each one of the plurality of consecutive cardiac events beingsensed as an R-wave in response to the cardiac electrical signalcrossing an R-wave sensing threshold; a therapy delivery circuitconfigured to deliver electrical stimulation therapy to a patient'sheart via electrodes coupled to the ICD; and a control circuitconfigured to: determine at least one sensed event parameter from thecardiac electrical signal for each one of the plurality of consecutivecardiac events sensed as an R-wave by the sensing circuit; compare thesensed event parameters to P-wave oversensing criteria; detect P-waveoversensing in response to the sensed event parameters meeting theP-wave oversensing criteria, the P-wave oversensing corresponding to atleast one P-wave being falsely sensed as at least one of the pluralityof consecutive cardiac events that is sensed as an R-wave by the sensingcircuit; and adjust at least one of an R-wave sensing control parameteror a therapy delivery control parameter in response to detecting theP-wave oversensing.
 2. The system of claim 1, wherein the controlcircuit is configured to: determine the at least one sensed eventparameter by determining event time intervals between the plurality ofconsecutive cardiac events; compare the event time intervals to at leasta first time interval threshold; detect a cluster of sensed events basedon at least a threshold number of the event intervals being less thanthe first time interval threshold and at least one of the eventintervals being greater than the first time interval threshold andconsecutive with the threshold number of event intervals ; and detectthe P-wave oversensing in response to detecting the cluster of sensedevents.
 3. The system of claim 2, wherein the control circuit is furtherconfigured to detect the P-wave oversensing by: comparing the cluster ofcardiac events to the P-wave oversensing criteria; and identifying thecluster of cardiac events as P-wave oversensing when the cluster ofcardiac events meets the P-wave oversensing criteria.
 4. The system ofclaim 2, wherein the control circuit is further configured to: determineif a threshold number of clusters of cardiac events is detected based ondetermining time intervals between sensed cardiac events; determine atleast one morphology feature of each the detected clusters of sensedevents in response to the threshold number of clusters of cardiac eventsbeing detected; compare the at least one morphology feature to P-waveoversensing criteria; and identify each cluster of cardiac events havingthe at least one morphology feature meeting the P-wave oversensingcriteria as P-wave oversensing.
 5. The system of claim 2, wherein thecontrol circuit is configured to: determine a maximum peak amplitude ofeach of the detected clusters of cardiac events; compare the maximumpeak amplitude to an amplitude threshold; and identify each detectedcluster of cardiac events having the maximum peak amplitude less thanthe amplitude threshold as P-wave oversensing.
 6. The system of claim 2,wherein the control circuit is further configured to: determine amorphology match score between each of the sensed events of eachdetected cluster of cardiac events and a previously determined R-wavetemplate; compare each morphology match score to a match scorethreshold; and identify each cluster of cardiac events having at least apredetermined portion of the determined morphology match scores for therespective cluster of cardiac events being less than a match scorethreshold.
 7. The system of claim 2, wherein the control circuit isfurther configured to: determine a peak amplitude from the cardiacelectrical signal for each of the plurality of consecutive cardiacevents; determine a morphology matching score for each of the pluralityof consecutive cardiac events by comparing the cardiac electrical signalto a pre-determined R-wave template; determine a first pattern of thepeak amplitudes determined for the consecutive cardiac events bylabeling each of the peak amplitudes as one of high or low based on acomparison of the peak amplitudes to an amplitude threshold; determine asecond pattern of the morphology matching scores determined for theconsecutive cardiac events by labeling each of the morphology matchingscores as one of match or non-match based on a comparison of themorphology matching scores to an R-wave matching threshold; determine athird pattern of the event time intervals determined for the consecutivecardiac events by labeling each of the event time intervals as one ofshort or long based on a comparison of the event time intervals to theevent interval threshold; and detect the cluster of sensed events basedon at least one of the first pattern, the second pattern and the thirdpattern being an alternating pattern.
 8. The system of claim 1, furthercomprising a memory coupled to the control circuit and a telemetrycircuit comprising a transceiver for transmitting and receiving datafrom an external medical device, the control circuit further configuredto store cardiac electrical signal data in the memory in response toidentifying the P-wave oversensing and control the telemetry circuit totransmit the stored cardiac electrical signal data to the externalmedical device.
 9. The system of claim 1, wherein the P-wave oversensingcriteria comprises a first set of P-wave oversensing criteria and asecond set of P-wave oversensing criteria, and the control circuit isconfigured to: detect and identify a first cluster of cardiac eventssensed by the sensing circuit based on the first set of P-waveoversensing criteria; detect and identify a second cluster of cardiacevents sensed by the sensing circuit based on the second set of P-waveoversensing criteria; control the therapy delivery circuit to provideone of a first therapy response in response to identifying the firstcluster of cardiac events or a second therapy response in response toidentifying the second cluster of cardiac events, the first therapyresponse different than the second therapy response.
 10. The system ofclaim 9, wherein the first therapy response comprises delivering atleast one bradycardia pacing pulse, and the second therapy responsecomprises withholding delivery of a tachyarrhythmia therapy.
 11. Thesystem of claim 1, wherein the control circuit is configured to: adjustthe therapy control parameter by enabling R-wave confirmation inresponse to identifying the P-wave oversensing; set a bradycardia pacingescape interval; compare the cardiac electrical signal to R-waveconfirmation criteria in response to the sensing circuit sensing acardiac event; reset the pacing escape interval in response toconfirming the sensed cardiac event as being an R-wave based oncomparing the cardiac electrical signal to the R-wave confirmationcriteria; allow the pacing escape interval to continue running inresponse to not confirming the sensed cardiac event as being an R-wavebased on comparing the cardiac electrical signal to the R-waveconfirmation criteria.
 12. The system of claim 1, wherein the controlcircuit is configured to: detect a tachyarrhythmia episode based on thecardiac electrical signal; reject the tachyarrhythmia episode detectionin response to identifying the P-wave oversensing.
 13. The system ofclaim 1, wherein the control circuit is further configured to: determinethe at least one sensed event parameter by determining a maximum peakamplitude of the cardiac electrical signals corresponding to eachcardiac event sensed by the sensing circuit; compare the sensed eventparameters to the P-wave oversensing criteria by comparing the maximumpeak amplitude determined for each sensed event to a maximum P-waveamplitude threshold; and detect the P-wave oversensing in response tothe maximum peak amplitude of a sensed cardiac event being less than theP-wave amplitude threshold.
 14. The system of claim 13, wherein thecontrol circuit is further configured to: set an interval timer inresponse to the sensing circuit sensing a cardiac event; reset theinterval timer in response to the maximum peak amplitude being greaterthan the maximum P-wave amplitude threshold; and allow the intervaltimer to continue running in response to the maximum peak amplitudebeing less than the maximum P-wave amplitude threshold.
 15. The systemof claim 14, wherein the control circuit is configured to set theinterval timer to a pacing escape interval and control the therapydelivery circuit to deliver a pacing pulse in response to the intervaltimer expiring.
 16. The system of claim 1, further comprising anextra-cardiovascular lead carrying at least one of the electrodescoupled to the ICD.
 17. A method comprising: receiving a cardiacelectrical signal; sensing a plurality of consecutive cardiac eventsfrom the cardiac electrical signal, each one of the plurality ofconsecutive cardiac events being sensed as an R-wave in response to thecardiac electrical signal crossing an R-wave sensing threshold;determining at least one sensed event parameter from the cardiacelectrical signal for each one of the plurality of consecutive sensedcardiac events; comparing the sensed event parameters to P-waveoversensing criteria; detecting P-wave oversensing in response to thesensed event parameters meeting the P-wave oversensing criteria, theP-wave oversensing corresponding to at least one P-wave being falselysensed as at least one of the plurality of consecutive cardiac eventsthat is sensed as an R-wave by the sensing circuit; and adjusting atleast one of an R-wave sensing control parameter or a therapy deliverycontrol parameter in response to detecting the P-wave oversensing. 18.The method of claim 17, further comprising determining the at least onesensed event parameter by determining event time intervals between theplurality of consecutive cardiac events; comparing the event timeintervals to at least a first time interval threshold; detecting acluster of sensed events based on at least a threshold number of theevent intervals being less than the first time interval threshold and atleast one of the event intervals being greater than the first timeinterval threshold and consecutive with the threshold number of eventintervals; and detecting the P-wave oversensing based on detecting thecluster of sensed events.
 19. The method of claim 18, further comprisingdetecting the P-wave oversensing by: comparing the cluster of cardiacevents to the P-wave oversensing criteria; and identifying the clusterof cardiac events as P-wave oversensing when the cluster of cardiacevents meets the P-wave oversensing criteria.
 20. The method of claim18, further comprising: determining if a threshold number of clusters ofcardiac events is detected based on determining time intervals betweensensed cardiac events; determining at least one morphology feature ofeach the detected clusters of sensed events in response to the thresholdnumber of clusters of cardiac events being detected; comparing the atleast one morphology feature to P-wave oversensing criteria; andidentifying each cluster of cardiac events having the at least onemorphology feature meeting the P-wave oversensing criteria as P-waveoversensing.
 21. The method of claim 18, further comprising: determininga maximum peak amplitude of each of the detected clusters of cardiacevents; comparing the maximum peak amplitude to an amplitude threshold;and identifying each detected cluster of cardiac events having themaximum peak amplitude less than the amplitude threshold as P-waveoversensing.
 22. The method of claim 18, further comprising: determininga morphology match score between each of the sensed events of eachdetected cluster of cardiac events and a previously determined R-wavetemplate; comparing each morphology match score to a match scorethreshold; and identifying each cluster of cardiac events having atleast a predetermined portion of the determined morphology match scoresfor the respective cluster of cardiac events being less than a matchscore threshold.
 23. The method of claim 17, further comprising: storingcardiac electrical signal data in a memory in response to identifyingthe P-wave oversensing; and transmitting the stored cardiac electricalsignal data to an external medical device.
 24. The method of claim 17,further comprising: storing in the memory the P-wave oversensingcriteria comprising a first set of P-wave oversensing criteria and asecond set of P-wave oversensing criteria; detecting and identifying afirst cluster of sensed cardiac events based on the first set of P-waveoversensing criteria; detecting and identifying a second cluster ofsensed cardiac events based on the second set of P-wave oversensingcriteria; providing one of a first therapy response in response toidentifying the first cluster of cardiac events or a second therapyresponse in response to identifying the second cluster of cardiacevents, the first therapy response different than the second therapyresponse.
 25. The method of claim 24, wherein the first therapy responsecomprises delivering at least one bradycardia pacing pulse, and thesecond therapy response comprises withholding delivery of atachyarrhythmia therapy.
 26. The method of claim 17, further comprising:wherein adjusting the therapy control parameter comprises enablingR-wave confirmation in response to identifying the P-wave oversensing;setting a bradycardia pacing escape interval; comparing the cardiacelectrical signal to R-wave confirmation criteria in response to sensinga cardiac event; resetting the pacing escape interval in response toconfirming the sensed cardiac event as being an R-wave based oncomparing the cardiac electrical signal to the R-wave confirmationcriteria; and allowing the pacing escape interval to continue running inresponse to not confirming the sensed cardiac event as being an R-wavebased on comparing the cardiac electrical signal to the R-waveconfirmation criteria.
 27. The method of claim 17, further comprising:detecting a tachyarrhythmia episode based on the cardiac electricalsignal; rejecting the tachyarrhythmia episode detection in response toidentifying the P-wave oversensing.
 28. The method of 17, furthercomprising: determining a peak amplitude from the cardiac electricalsignal for each of the plurality of consecutive cardiac events;determining a morphology matching score for each of the plurality ofconsecutive cardiac events by comparing the cardiac electrical signal toa pre-determined R-wave template; determining a first pattern of thepeak amplitudes determined for the consecutive cardiac events bylabeling each of the peak amplitudes as one of high or low based on acomparison of the peak amplitudes to an amplitude threshold; determininga second pattern of the morphology matching scores determined for theconsecutive cardiac events by labeling each of the morphology matchingscores as one of match or non-match based on a comparison of themorphology matching scores to an R-wave matching threshold; determininga third pattern of the event time intervals determined for theconsecutive cardiac events by labeling each of the event time intervalsas one of short or long based on a comparison of the event timeintervals to the event interval threshold; and detecting the cluster ofsensed events based on at least two of the first pattern, the secondpattern and the third pattern being an alternating pattern.
 29. Themethod of claim 17, further comprising: determining the at least onesensed event parameter by determining a maximum peak amplitude of thecardiac electrical signals corresponding to each sensed cardiac event;comparing the sensed event parameters to the P-wave oversensing criteriaby comparing the maximum peak amplitude determined for each sensed eventto a maximum P-wave amplitude threshold; and detecting the P-waveoversensing in response to the maximum peak amplitude of a sensedcardiac event being less than the P-wave amplitude threshold.
 30. Themethod of claim 29, further comprising: setting an interval timer inresponse to a cardiac event; resetting the interval timer in response tothe maximum peak amplitude being greater than the maximum P-waveamplitude threshold; and allowing the interval timer to continue runningin response to the maximum peak amplitude being less than the maximumP-wave amplitude threshold.
 31. The method of claim 30, wherein settingthe interval times comprises setting the interval timer to a pacingescape interval, the method further comprising delivering a pacing pulsein response to the interval timer expiring.
 32. A non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control circuit of a medical system, cause the systemto: receive a cardiac electrical signal by a sensing circuit viaelectrodes coupled to the medical system; sense a plurality ofconsecutive cardiac events from the cardiac electrical signal, each oneof the plurality of consecutive cardiac events being sensed as an R-wavein response to the cardiac electrical signal crossing an R-wave sensingthreshold; determine by a control circuit of the medical system at leastone sensed event parameter from the cardiac electrical signal for eachone of the plurality of consecutive cardiac events sensed as an R-waveby the sensing circuit; compare the sensed event parameters to P-waveoversensing criteria; detect P-wave oversensing in response to thesensed event parameters meeting the P-wave oversensing criteria, theP-wave oversensing corresponding to at least one P-wave being falselysensed as at least one of the plurality of consecutive cardiac eventsthat is sensed as an R-wave by the sensing circuit; and adjust at leastone of an R-wave sensing control parameter or a therapy delivery controlparameter in response to detecting the P-wave oversensing.